P0304 Code: Cylinder 4 Misfire Detected

P0304 Code — Cylinder 4 Misfire Detected

P0304 means the powertrain control module (PCM) has detected a misfire event specifically on cylinder 4. This is not a guess — the PCM knows it is cylinder 4 because it monitors crankshaft rotational speed variation on a cylinder-by-cylinder basis using the crankshaft position (CKP) sensor signal. Each time a cylinder fires correctly, it adds a power pulse to the crankshaft. When cylinder 4 misfires, that pulse is absent. The crankshaft decelerates slightly during that firing event, and the PCM measures the time between CKP reluctor wheel tooth signals to detect the deceleration. When the deviation exceeds the calibrated threshold for that RPM and load range, P0304 sets.

P0304 stores as a pending code first. If the misfire rate crosses the catalyst damage threshold on two consecutive drive cycles, it confirms and the MIL illuminates. If the misfire is severe enough to damage the catalytic converter in real time, the MIL will flash — that is your signal to stop the vehicle. A flashing MIL from a misfire can destroy a catalytic converter in minutes at highway speed. Address P0304 quickly. The converter replacement cost will dwarf the cost of whatever caused the misfire in the first place.

Why Cylinder 4 — Platform-Specific Problems

Cylinder 4 is not always the problem child, but on several high-volume platforms it has a documented history of being the first or most frequent misfire cylinder. Understanding the platform-specific reasons saves you diagnostic time before you ever connect a scan tool.

Subaru EJ-Series — Head Gasket Failure Targeting Cylinder 4

The Subaru EJ-series engine — particularly the 2.5L SOHC EJ253 found in Foresters, Outbacks, and Impreza wagons from the late 1990s through the mid-2000s — has a well-documented head gasket failure pattern. The EJ-series uses a composite head gasket with a graphite sealing layer, and that material degrades over time when exposed to thermal cycling and Subaru's cooling system chemistry. The failure typically begins at the external water jacket sealing area, but as it progresses it can breach internally between the coolant passage and the combustion chamber.

Cylinder 4 is the rearmost cylinder on the passenger-side bank of the EJ horizontally-opposed engine. Due to coolant flow dynamics and heat management in the EJ design, cylinder 4 is typically the first to show signs of coolant intrusion. A small amount of coolant entering the combustion chamber will cause a misfire — particularly at cold start when the engine has not yet built enough heat to flash the intrusion to steam quickly. Freeze frame data on a Subaru EJ with P0304 often shows the misfire occurring at cold start or low load, with coolant temperature below normal operating range.

The diagnostic flags specific to Subaru EJ head gasket failure on cylinder 4 are: a spark plug that shows white, crusty, or steam-cleaned deposits; a positive combustion leak test (block test or exhaust gas analyzer sniffed at the radiator); coolant level that drops slowly without an external leak; and a cylinder 4 compression reading that is borderline or trending low without a wet test improvement. Do not let the absence of milky oil fool you — internal coolant intrusion at cylinder 4 on the EJ can occur at the fire ring or the external water jacket seam without mixing with oil at all. The combustion leak test is your primary confirmation tool on this platform.

Honda K-Series — Carbon Buildup on GDI Variants

Honda's K-series engines span a long production run across the Civic, CR-V, Accord, and Acura platforms. The earlier K-series engines use port fuel injection and are generally robust. The issue with cylinder 4 on K-series and K-series-derived engines arises on the newer variants that use gasoline direct injection (GDI) — including the 1.5L L15B7 turbocharged four-cylinder found in the 10th and 11th generation Civics and several CR-V models.

GDI engines inject fuel directly into the combustion chamber, bypassing the intake valve entirely. This means the intake valve never gets washed with fuel during normal operation. Oil vapor circulated through the PCV system deposits carbon on the intake valve stems and backsides over time. On four-cylinder GDI engines, cylinder 4 is at the end of the intake manifold runners and can accumulate deposits differently than cylinders 1 through 3 depending on PCV routing and manifold geometry. When carbon buildup restricts the intake valve on cylinder 4, airflow into that cylinder is reduced and a lean misfire develops — most prominently at idle and light throttle where airflow velocity is lowest.

The diagnostic signature on a Honda GDI carbon misfire is that P0304 often appears alone or with an adjacent cylinder, the vehicle runs noticeably better at RPM above 2,000 where airflow velocity overcomes the restriction, and a borescope inspection of the cylinder 4 intake port will show clearly visible carbon deposits on the valve backsides. The fix is walnut shell blasting of the intake ports — intake manifold removal required, media blasting setup required, expect 3 to 4 hours on this platform.

GM LS-Series and 5.3L V8 — AFM/DOD Lifter Collapse on Cylinder 4

On GM's 5.3L and 6.2L V8 engines equipped with Active Fuel Management (AFM) — also called Displacement on Demand (DOD) on earlier models — the system deactivates selected cylinders at light load by collapsing the valve lifters on those cylinders. The deactivation cylinders on the Gen IV 5.3L include cylinders 1, 4, 6, and 7. Cylinder 4 is one of the four cylinders that uses the collapsible AFM lifter design.

The AFM lifter mechanism is oil-pressure dependent. A solenoid routes pressurized oil to the lifter, which allows the inner plunger to collapse. When the solenoid releases oil pressure, the lifter should re-extend and return the cylinder to active status. When the lifter body fails — due to wear, contaminated oil, or material failure — the plunger collapses and does not re-extend. The cylinder is now stuck in a deactivated state. It has no valve lift, no compression, and no combustion contribution. P0304 sets, often alongside P0300 because the severity of the cylinder 4 dropout is enough to trip the random misfire monitor as well.

On the 5.3L, cylinder 4 is on bank 2 (passenger side on most configurations). The diagnostic confirmation for AFM lifter failure on cylinder 4 is a relative compression test showing near-zero contribution from that cylinder, followed by a cylinder contribution (balance) test with the same result. Pulling the valley cover and inspecting the AFM lifters on cylinder 4 will reveal the collapsed or fractured lifter body. The repair is an AFM delete kit — replacement lifters for all eight cylinders, camshaft on affected variants, valley cover delete plate, and an AFM disabler if the customer wants to prevent recurrence without a full tune. Do not replace just the cylinder 4 lifter — the other AFM lifters are likely at similar wear state and will fail shortly.

Ignition System Causes

Ignition faults are the most common cause of a single-cylinder misfire code across all platforms. Start here unless the data clearly points elsewhere. The components to evaluate are the spark plug, the coil-on-plug (COP) assembly or distributor cap and rotor on older vehicles, and the plug boot if the coil and plug are separate units.

Spark Plug Inspection

Pull the cylinder 4 plug and read it before doing anything else. The plug condition tells you what is happening inside that combustion chamber. Look for:

• Worn electrode gap — an oversized gap raises the voltage required to fire the plug. If the coil cannot supply that voltage consistently, you get misfires. Measure the gap. Compare to spec. On extended-life iridium or platinum plugs that are at or past service interval, gap wear alone can cause P0304.
• Oil fouling — wet, oily deposits on the plug indicate oil intrusion into the combustion chamber. Do not just replace the plug. Find the oil source — worn rings, valve stem seals, or a failed head gasket allowing oil passage crossover — before the new plug fouls in the same way.
• Carbon fouling — dry, matte black deposits suggest a persistent rich condition or a plug running too cold. Check for a stuck-open injector on cylinder 4, high fuel pressure, or a vehicle that never gets driven hard enough to clean the plug self-clean temperature.
• Coolant fouling — white, crusty, or steam-cleaned deposits are a head gasket red flag. Go straight to a combustion leak test before spending time on ignition components.
• Cracked insulator or damaged electrode — physical damage from detonation, incorrect plug installation torque, or plug threads seizing in an aluminum head.

COP Coil Swap Test

The coil swap test is mandatory before condemning any COP coil. Move the cylinder 4 coil to a known-good cylinder — cylinder 2 or 5 work well depending on the engine layout — and clear the codes. Drive or idle the vehicle through the condition where the misfire occurs. If the misfire follows the coil to the new cylinder and sets a code for that cylinder instead of P0304, the coil is the problem. If P0304 stays on cylinder 4 after the swap, the coil is not the root cause.

This test costs nothing but time and eliminates or confirms the coil without buying a part first. A coil that fails the swap test is a confirmed component failure. A coil that passes the swap test sends you to fuel or mechanical diagnosis with confidence.

When you have the coil off, inspect the boot and boot seal. A cracked or torn boot allows the high-voltage spark to arc to the valve cover or cylinder head rather than jumping the plug gap. This produces a misfire that is often intermittent and worse in high humidity or wet conditions. Run a bright light along the boot and look for cracks, tears, or carbon tracking on the exterior of the boot where arc-over has occurred.

Fuel System Causes

If the swap test rules out ignition and the plug looks clean, move to fuel. A single-cylinder fuel fault almost always means the injector rather than the fuel pressure supply, but confirm system fuel pressure first — especially under load — to eliminate a weak pump or failing pressure regulator from consideration.

Injector Failure and Cylinder Contribution Testing

A stuck-closed or severely clogged injector on cylinder 4 will deliver insufficient fuel, producing a lean misfire. A stuck-open injector will deliver too much fuel, producing a rich misfire and potentially washing the oil film off the cylinder 4 bore on port injection systems — which causes rapid ring wear if left unaddressed.

On port injection systems, perform a cylinder contribution test. The scan tool cuts fuel to each injector one at a time and monitors the RPM drop. A functioning cylinder will drop RPM noticeably when its injector is cut because the engine loses a contributing cylinder. A cylinder that is already not contributing due to a failed injector will show little or no RPM drop when its injector is cut — because cutting it changes nothing. No RPM drop when cylinder 4's injector is cut confirms a delivery problem at that injector.

On GDI systems, the injector operates at high pressure — 200 bar or more on some systems — and cannot be tested the same way. Compare the commanded pulse width for the cylinder 4 injector against the other cylinders at the same operating condition using live scan data. A significant deviation suggests the PCM is compensating for a fuel delivery problem at cylinder 4, or the injector is not responding to commands correctly. Some platforms allow individual injector disable commands through the scan tool for GDI systems — use that function if available.

Fuel Pressure Drop Under Load

If multiple cylinders show elevated misfire counts in Mode $06 but cylinder 4 is highest, consider fuel pressure drop under load rather than a cylinder-specific component failure. A weak fuel pump that maintains adequate pressure at idle but cannot keep up under acceleration will cause misfires that appear to concentrate on one or two cylinders — often the ones farthest from the fuel pressure regulator or at the end of the fuel rail. Connect a fuel pressure gauge and monitor pressure during a load test. A significant pressure drop at wide-open throttle or under sustained load confirms pump or pressure regulator issues.

Mechanical Causes

Mechanical misfires are the most expensive to diagnose and repair. Pursue them after ignition and fuel have been ruled out, or when the data points directly to a mechanical cause from the start — low relative compression, a known-bad platform, or physical evidence on the spark plug.

Compression Loss and Valve Issues

Low compression on cylinder 4 will produce a misfire that no ignition or fuel repair will resolve. The compression cannot be restored by replacing components in those systems. Common causes of compression loss on cylinder 4 include:

• Worn piston rings — compression is low and improves when oil is added to the cylinder for a wet compression test. The oil temporarily seals the ring gap.
• Burned or bent exhaust valve — compression is low and does not improve wet. A leakdown test will show air escaping at the exhaust pipe.
• Burned or bent intake valve — compression is low and does not improve wet. Leakdown shows air escaping back through the intake manifold.
• Valve seat recession — common on older aluminum head designs or engines that ran without lead-tolerant valve seat inserts. The seat recedes into the head, reducing valve sealing over time.

Start with a relative compression test via a scan tool that reads CKP signal variation during cranking. This gives you a fast comparison across all cylinders without removing plugs. If cylinder 4 is significantly lower than the others, follow up with a standard compression test, then a wet compression test, then a cylinder leakdown test to identify exactly where the pressure is going. Each test narrows the repair path.

Head Gasket Breach Between Cylinders 3 and 4

On inline four-cylinder engines, cylinders 3 and 4 are adjacent and share a section of the head gasket. A head gasket breach between cylinders 3 and 4 is one of the most common inter-cylinder failure points. When the gasket fails at the fire ring between these two cylinders, compression can leak from one cylinder to the other, causing both to lose compression simultaneously. You may see P0303 and P0304 set together, with both cylinders showing low compression on a standard test.

A leakdown test on cylinder 4 with cylinder 3 at the same TDC position will show air escaping into cylinder 3 — you can hear it through the open cylinder 3 plug hole. A standard compression test showing two adjacent cylinders both low is highly suggestive of a head gasket breach between them, even before the leakdown confirms it. If the breach also contacts a coolant passage, the combustion leak test will be positive and you may see coolant consumption without external leakage.

Diagnostic Approach — Step by Step

Work this code in order. Jumping to parts replacement without working the process leads to misdiagnosis and callbacks.

1. Pull freeze frame data first. Note engine load, RPM, coolant temperature, and fuel trim values at the moment P0304 set. A misfire that sets at cold start with low coolant temp points toward a different cause than one that sets at highway cruise under load. Positive short-term fuel trim corrections on cylinder 4 suggest a lean condition — air leak or fuel delivery problem. Negative fuel trims suggest rich — stuck-open injector or excessive fuel pressure.
2. Check Mode $06 misfire counters for all cylinders. Compare the misfire event count on cylinder 4 against every other cylinder. If cylinder 4 is dramatically higher and all others are near zero, you have an isolated component failure on that cylinder. If all cylinders show elevated counts with cylinder 4 highest, you are likely looking at a base engine or fuel system issue affecting the whole engine — not a component swap situation.
3. Inspect the cylinder 4 spark plug. Read the deposits, measure the gap, look for physical damage or contamination. The plug tells a story — read it before doing anything else.
4. Perform the COP coil swap test. Move the cylinder 4 coil to a known-good cylinder. Clear codes. Reproduce the operating condition. Let the data tell you if the fault moved with the coil.
5. Run a cylinder contribution test on port injection systems. Confirm injector delivery at cylinder 4. Skip fuel pressure monitoring only if the contribution test is clean and Mode $06 shows cylinder 4 as the only elevated cylinder.
6. Perform a relative compression test. Use a scan tool with CKP-based relative compression capability. This catches mechanical problems before you invest time in ignition and fuel diagnosis on a cylinder that cannot fire correctly regardless of component condition.
7. If relative compression is low, follow up with standard compression, wet compression, and leakdown. These three tests together will tell you the failure mode and point the repair in the right direction.

P0304 With P0300 — What That Combination Means

When P0304 appears alongside P0300 (random misfire detected), the combination has diagnostic significance beyond just two codes on the screen.

P0300 sets when the total misfire rate across the engine — or misfire events spread across multiple cylinders — crosses the catalyst damage monitoring threshold. When P0304 and P0300 appear together, it typically means one of two things: the cylinder 4 misfire is so severe and sustained that the total engine misfire rate has also tripped the P0300 threshold, or other cylinders are also misfiring at a lower rate that contributes to a combined total misfire rate that crosses the P0300 threshold.

On GM 5.3L AFM trucks, P0304 with P0300 is nearly diagnostic for AFM lifter failure on cylinder 4. The complete deactivation of a cylinder produces enough misfire severity to trip both monitors simultaneously. On any platform, this code combination suggests the misfire is not mild or intermittent — you are dealing with a significant failure that warrants prioritizing mechanical diagnosis early in the process rather than spending time on plug and coil swaps first.

Check Mode $06 misfire counters for all cylinders when you see this combination. If cylinder 4 is hundreds of misfire events above all other cylinders and the other cylinders are clean, cylinder 4 is causing both codes. If multiple cylinders show elevated counts, broaden your diagnostic approach to cover shared systems — fuel pressure, timing, EGR.

Catalytic Converter Damage From Sustained Misfire

This is not a theoretical concern. Unburned fuel from a misfiring cylinder 4 exits into the exhaust stream. That fuel reaches the catalytic converter while it is hot and oxidizes rapidly inside the catalyst substrate. The exothermic reaction can raise the catalyst temperature far beyond its design limits. At extreme temperatures, the catalyst substrate melts and fuses — creating flow restriction inside the converter that compounds the engine's problems and adds thousands of dollars to the repair bill.

A flashing MIL means the misfire rate is severe enough that catalyst damage is occurring right now, not just in a future drive cycle. When a customer comes in with a flashing MIL and P0304, the first conversation is about not driving the vehicle until the misfire is repaired. The second conversation is about inspecting the converter after the misfire repair is complete — a catalyst that has been subjected to a severe misfire for any significant period may already be damaged even if it is not completely melted. A pressure drop test across the converter or an infrared temperature comparison between the front and rear face of the converter will show you whether the substrate has been compromised.

Wiring and Connector Issues Specific to Cylinder 4

Cylinder 4's position on the engine varies by platform, and its location can make it more or less susceptible to specific wiring and connector problems.

On inline four-cylinder engines, cylinder 4 is the rearmost cylinder, typically closest to the firewall. This position makes the cylinder 4 coil connector and injector connector the hardest to reach for service — and in many cases the most exposed to heat from the exhaust manifold at the rear of the engine. Inspect the cylinder 4 COP connector body for heat damage, melted plastic, or cracked connector locks. Pull the connector and look at the terminals for corrosion, pushed-back pins, or spread terminals that are not making full contact.

On V6 and V8 engines, cylinder 4's position depends on the manufacturer's numbering convention. On GM V8s, cylinder 4 is the rearmost cylinder on bank 1 (driver side on most applications). On those platforms, check the cylinder 4 coil connector and the injector connector for corrosion from water intrusion — the rear of the driver-side bank can collect moisture in certain engine bay configurations.

If wiring is suspected, scope the coil primary circuit on cylinder 4 and compare the waveform against a known-good cylinder at the same operating condition. The coil primary should show a clean inductive spike pattern. A missing or degraded signal with a good coil installed points toward the PCM driver circuit or the wiring between the PCM and the coil. A scope test here prevents misdiagnosis of a wiring fault as a coil fault.

Common Misdiagnosis Pitfalls

These are the diagnostic mistakes that result in callbacks, unhappy customers, and lost shop time.

• Replacing the cylinder 4 coil without performing a swap test. If the coil is not the cause, the code comes back and the customer pays for a part they did not need. The swap test is free. There is no excuse for skipping it.
• Replacing plugs and coil on a Subaru EJ with white fouling on the cylinder 4 plug. White plug deposits on a Subaru EJ with P0304 need a combustion leak test before ignition components are sold. Replacing the plug does not fix a head gasket. The new plug will foul the same way within a few hundred miles.
• Missing AFM lifter failure on a GM 5.3L because compression tested at a low-normal number. A collapsing AFM lifter that is partially re-extending can show a low but not obviously failed compression number on a static test. The relative compression test and cylinder contribution test are more diagnostic on these engines. If the numbers do not add up, pull the valley cover.
• Not checking fuel trim data before chasing ignition components on a GDI engine. A lean misfire from carbon-restricted intake valves and a lean misfire from a failing injector both set P0304. Fuel trim data and a borescope inspection point you to the right repair before you start disassembling components.
• Clearing codes and returning the vehicle without reproducing the original misfire condition. If the freeze frame shows the misfire at highway cruise under load, idling in the bay after the repair does not confirm it is fixed. Drive it. Put it under load. Make sure P0304 does not return under the same conditions shown in the freeze frame data.
• Assuming P0304 with P0300 always means two separate problems. In most cases, a severe cylinder 4 misfire is causing both codes. Do not treat them as independent faults until Mode $06 data tells you that other cylinders are also misfiring at significant rates.

Three Real Shop Scenarios

Scenario 1 — 2004 Subaru Outback 2.5L EJ253, P0304 at Cold Start

Customer comes in with a rough idle at cold start and P0304 stored. The misfire goes away after the engine warms up. Freeze frame shows the code set at cold idle with coolant temperature at 38°F and 750 RPM. Tech pulls the cylinder 4 plug — it shows white, crusty deposits on the electrode and insulator. Tech performs a combustion leak test: positive result at the radiator with the engine warm. Standard compression test shows cylinder 4 at 135 PSI versus 165 to 170 PSI on cylinders 1 through 3. Leakdown on cylinder 4 shows air escaping into the cooling system. Diagnosis: head gasket failure with coolant intrusion into cylinder 4. Repair: head gasket replacement, cylinder head resurfacing, full cooling system flush. Code does not return. Total time: 9 hours including diagnosis.

Scenario 2 — 2018 Silverado 5.3L, P0304 and P0300 at Light Throttle

Customer reports check engine light on for two weeks, occasional rough running at highway cruise. Pulls codes: P0304 and P0300. Freeze frame shows codes set at 1,800 RPM, 25% load, light throttle. Fuel trims are normal. Tech performs a relative compression test — cylinder 4 shows near-zero contribution. Cylinder contribution test confirms cylinder 4 is not contributing. Tech removes the valley cover and inspects the cylinder 4 AFM lifters — one intake lifter has collapsed and fractured internally. Repair: full AFM delete kit including all eight cylinder lifters, valley cover delete plate, and a calibration update to disable AFM in the PCM. No return codes. Total time: 7 hours including diagnosis.

Scenario 3 — 2017 Honda Civic 1.5L Turbo, P0304 at Idle, Clears at RPM

Customer reports P0304, rough idle, no issues above 1,500 RPM. Tech swaps coil — code stays on cylinder 4. Plug looks acceptable. Cylinder contribution test shows reduced contribution from cylinder 4 at idle. Tech performs a borescope inspection of the cylinder 4 intake port — heavy carbon buildup on the valve backside, visible restriction of the port opening. No carbon present in the combustion chamber or exhaust side. Diagnosis: GDI carbon buildup on cylinder 4 intake valve. Repair: intake manifold removed, walnut shell blasting performed on all four intake ports with cylinder 4 cleaned most aggressively. Idle smooths out immediately after reassembly. P0304 does not return through two full drive cycles. Total time: 4 hours.

Summary

P0304 is specific and actionable. The PCM has already done the work of identifying cylinder 4 as the problem cylinder. Your job is to determine which system — ignition, fuel, or mechanical — is failing to deliver a complete combustion event in that cylinder. Start with freeze frame data and Mode $06 misfire counters to understand the severity and operating condition. Read the cylinder 4 spark plug before replacing anything. Perform the coil swap test before condemning any COP coil. On Subaru EJ platforms, treat coolant-fouled plugs as a head gasket problem until proven otherwise. On GM 5.3L AFM trucks, treat P0304 with P0300 as a potential lifter failure and check relative compression early. On GDI engines with idle-only misfires, borescope the intake port before pulling injectors. Address P0304 promptly — every mile driven with an active misfire is feeding unburned fuel into the catalytic converter. The misfire repair is always cheaper than the converter replacement.