P0304: Cylinder 4 Misfire — Diagnosis and Causes

P0304 — cylinder 4 misfire detected. On an inline-4 engine, cylinder 4 is the last cylinder in the bank and the one closest to the firewall and exhaust. On GM V8s with Active Fuel Management (AFM), cylinder 4 is one of the deactivation cylinders — and that matters a lot diagnostically.

Standard swap test methodology still applies, but this article focuses on the compression and mechanical side of cylinder 4 misfires, plus the AFM angle that makes this code unique on GM trucks.

What This Code Actually Means

The PCM detected repeated misfire events on cylinder 4's power stroke. Same crankshaft acceleration monitoring as P0301–P0303. The code sets when the misfire count exceeds the calibrated threshold. Flashing MIL = catalytic converter damage risk.

Common Causes

• Ignition coil or spark plug: Always rule this out first with a swap test.
• Fuel injector: Clogged, electrically open, or leaking. Test as outlined in the P0302 article.
• Low compression: This is where P0304 gets interesting — more below.
• GM AFM lifter failure: On GM 5.3L and 6.2L V8s, cylinder 4 is a deactivation cylinder. Failed AFM lifters are the number one pattern failure for P0304 on these engines.
• Exhaust restriction: On inline-4 engines, cylinder 4 is closest to the exhaust and most affected by backpressure. A plugged catalytic converter hits cylinder 4 hardest.
• Head gasket breach: On Subaru EJ25 boxer engines, cylinder 4 is the rear-left cylinder and is a common location for head gasket failure. Coolant intrusion causes misfire.

Diagnostic Approach: Compression and Mechanical Focus

Step 1: Swap Test First

Do the coil and plug swap test. If the misfire follows the coil or plug, you are done. If it does not, continue below.

Step 2: Compression Test

Run a compression test on all cylinders. You are comparing cylinder 4 to the rest. Spec for most engines is 125–180 PSI depending on compression ratio, but the number that matters most is the variance — more than 10% below the average is a concern.

What low compression on cylinder 4 tells you:

• Below spec, wet test improves it by 20+ PSI: Ring wear or damage on cylinder 4. Rings are not sealing.
• Below spec, wet test does not change it: Valve issue — burned exhaust valve, broken valve spring, or bent valve.
• Below spec on cylinders 3 AND 4: Possible head gasket breach between the two cylinders. Run a leak-down test and check for bubbles in the adjacent cylinder.

Step 3: Leak-Down Test

Apply regulated shop air (80–100 PSI) to cylinder 4 through the spark plug hole with the piston at TDC on the compression stroke. Acceptable leakage is under 20%. Listen and observe:

• Air at the intake: Intake valve not seating.
• Air at the tailpipe: Exhaust valve not seating (most common single-valve failure).
• Air at the oil filler cap: Ring blow-by.
• Bubbles in coolant: Head gasket.
• Air in adjacent cylinder: Head gasket breach between cylinders.

Step 4: Exhaust Backpressure Check (Inline-4)

On inline-4 engines, if all four cylinders have slightly low compression or the engine seems down on power across the board, check exhaust backpressure. Thread a pressure gauge into the upstream O2 sensor bung. At 2,500 RPM, backpressure should be under 1.5 PSI on most engines. Above 3 PSI indicates a restricted catalytic converter. Cylinder 4, being closest to the exhaust manifold collector and cat, feels this restriction the most and will misfire first.

GM AFM / Cylinder Deactivation Misfire

This deserves its own section because it is the most common P0304 pattern failure in the field right now.

On GM 5.3L (L83, L84, L82) and 6.2L (L87, LT4) V8s with Active Fuel Management, cylinders 1, 4, 6, and 7 are the deactivation cylinders. When AFM activates under light load, these cylinders' intake and exhaust valves are held closed by special AFM lifters with oil-pressure-operated locking mechanisms.

The failure mode: an AFM lifter collapses or sticks in the deactivated position. The valve does not open when it should, cylinder 4 gets no air, no combustion occurs, and you get a misfire. The telltale sign is a misfire that only occurs during highway cruising at light throttle — exactly when AFM is active.

How to confirm: Use your scan tool to monitor AFM status. Command AFM off (if your tool supports it) or simply drive the vehicle under conditions that keep all 8 cylinders active (moderate to heavy throttle). If the misfire disappears when AFM is not active, the AFM lifter is the cause.

Common TSBs and Pattern Failures

• GM 5.3L/6.2L V8 with AFM: TSB 21-NA-185 addresses AFM lifter failures. Revised lifters are available. Many shops are now doing full AFM delete kits (non-AFM camshaft, standard lifters, valley cover, and a tune to disable AFM) as a permanent solution. GM also released Dynamic Fuel Management (DFM) on newer models — different system, same category of potential issues.
• 2014–2023 GM Silverado/Sierra 5.3L: P0304 on cylinder 4 during highway driving is one of the most common complaints on these trucks. Check for the characteristic ticking noise at idle that accompanies a collapsed lifter.
• Subaru EJ25 (2.5L SOHC — Outback, Legacy, Forester, Impreza): Head gasket failure on the rear cylinders (including cylinder 4) is well documented. External coolant leak or combustion gas in coolant are the indicators. This typically occurs between 100,000 and 150,000 miles on original gaskets.
• Honda K-Series (2.0L, 2.4L — Civic, Accord, CR-V): Exhaust valve recession on high-mileage engines can cause low compression on cylinder 4. Check valve lash — if cylinder 4's exhaust valve clearance is significantly different from spec, the valve or seat is worn.

Cylinder 4 misfires are straightforward on most engines — the swap test handles the ignition side, and compression testing handles the mechanical side. The exception is GM AFM trucks, where the deactivation system adds a whole layer of diagnostic logic. If you are working a P0304 on a GM V8 and need help confirming whether AFM is the culprit, APEX Tech's AI Diagnostics can walk through the scan data with you.