Crankshaft and Camshaft Position Sensors — What Fails and How to Find It

Why These Sensors Are Foundational

The crankshaft position sensor is arguably the most critical sensor on the entire engine. Without crankshaft position data, the PCM has no reference point for where the pistons are in their travel. Without piston position, it cannot calculate when to fire the injectors or the ignition coils. Everything else in the engine management system — MAF sensor, coolant temperature, oxygen sensors, fuel trims — is meaningless without the timing reference that only the CKP sensor provides.

The camshaft position sensor is nearly as critical. While the engine can theoretically run without cam position data (the PCM can infer cylinder sequence from the crank pattern with some engines), it cannot properly control variable valve timing (VVT), cannot perform sequential injection (must default to batch firing), and may not be able to identify individual cylinders for precise misfire detection. Many engines require both CKP and CMP signals to start.

When either sensor fails completely, the result is immediate and dramatic — usually a no-start condition or a sudden stall. When either sensor fails intermittently, the result is equally dramatic but harder to catch — the engine runs fine 95% of the time and suddenly dies, then restarts as if nothing happened. These intermittent stall complaints are among the most frustrating diagnostic scenarios, and CKP and CMP sensor failures are a leading cause.

Hall Effect vs Variable Reluctance

Hall effect sensors (also called magnetic switch sensors or digital sensors) are three-wire active devices. One wire carries a 5-volt reference from the PCM, one wire is signal ground, and one wire returns the signal to the PCM. As the metal teeth of the reluctor ring pass the sensor face, the Hall effect semiconductor switches the output voltage — near ground (0.2-0.5V) when a tooth is present, near reference voltage (4.5-4.9V) when the gap between teeth is present. The result is a clean, square wave that the PCM can read at any speed, including very slow cranking. Hall effect sensors do not generate their own voltage — they require the 5V reference to operate.

Variable reluctance sensors (also called magnetic induction sensors or passive sensors) are two-wire passive devices. They contain a permanent magnet with a coil of wire wound around it. As a metal tooth on the reluctor ring approaches the sensor face, it changes the magnetic flux through the coil, inducing a small AC voltage. As the tooth passes the sensor face and moves away, the flux change reverses, producing the negative half of the sine wave. The signal is an AC sine wave whose amplitude and frequency both increase proportionally with engine speed. At cranking speed (200-300 RPM), the signal may be only 1-2 volts amplitude. At running speed (1,000 RPM), it may be 10-15V. VR sensors cannot reliably produce signal at very low speeds — this is why some vehicles with VR crankshaft sensors have a brief crank-before-start behavior while the PCM waits for adequate sensor signal to establish reference.

Testing differs between the two types. A Hall effect sensor is tested by checking for 5V reference, ground, and observing the switching signal with a DVOM or scope. A VR sensor is tested by checking coil resistance (typically 500-2,000 ohms — see service data) and checking for the AC voltage signal while cranking. A VR sensor with correct resistance but no signal during cranking may have an air gap issue (sensor backed out too far from the reluctor ring) or a problem with the reluctor ring itself.

The Reluctor Ring

The reluctor ring (also called trigger wheel or tone wheel) is a toothed metal ring that provides the reference pattern the position sensors read. On most modern engines, the crankshaft reluctor ring has either 36-1 or 60-2 teeth patterns — 36 or 60 evenly spaced teeth with one or two teeth intentionally missing. The missing teeth create a reference gap that the PCM uses to identify TDC on a specific cylinder on each crankshaft revolution.

On a 36-1 ring, the PCM counts 35 evenly spaced tooth signals and then sees a gap (the missing 37th tooth position). That gap always occurs at the same crank angle relative to TDC on cylinder 1, giving the PCM its absolute position reference on every crankshaft revolution. The camshaft signal provides cylinder identification so the PCM knows which revolution is the compression stroke vs the exhaust stroke for cylinder 1.

Reluctor ring damage causes intermittent or consistent position signal errors. Physical damage (a tooth broken off by a loose timing chain) removes one tooth signal from the expected pattern — the PCM sees what appears to be a larger gap than the reference gap, misidentifies position, and either sets a DTC or misfires on the cylinders around that position. A reluctor ring that has shifted on the crankshaft changes the angular relationship between the reference gap and actual TDC — the PCM fires injectors and ignition at the wrong piston positions, causing running issues and correlation codes.

Crankshaft Sensor Failure Patterns

Crankshaft position sensor failures follow a recognizable pattern that helps distinguish them from other no-start and stall causes. The most characteristic pattern is the intermittent hot stall: the vehicle runs fine, then suddenly cuts out completely while driving, often without any warning (no hesitation or gradual degradation). The engine restarts after a few minutes of cooling. This pattern — immediate stall, normal restart after cool-down — points strongly to a thermally failing CKP sensor. The sensor fails when hot and recovers when cooled. It may be a weak Hall effect circuit, a cracked sensor body that causes intermittent signal loss, or a failing internal magnet on a VR sensor.

Another pattern is a no-start that occurs randomly — the car starts 95 times without issue and then will not start on the 96th crank. If the sensor fails at rest during the last cool-down cycle and does not recover, the next crank produces no signal and no start. These cases are difficult because the symptom cannot be reproduced in the shop, the car often starts normally when the tech cranks it, and there may be no stored codes if the failure was brief.

For suspected intermittent CKP sensor failure: check for stored misfire codes and pending codes. Even if no CKP-specific code is present, intermittent misfire counts clustered around a specific crank position can indicate reluctor ring or sensor signal issues. A freeze frame showing RPM dropping to zero immediately before a MIL illumination event is a strong indicator of sudden sensor signal loss.

Camshaft Sensor Failure Patterns

Camshaft position sensor failure is often less dramatic than CKP failure because on most engines, the PCM can continue running the engine in a degraded mode without cam position data — it defaults to batch injection (firing all injectors together rather than sequentially) and loses VVT control. The engine may run rough, produce more emissions, and lose some efficiency, but it keeps running.

Symptoms of CMP sensor failure include: a P0340-P0349 range code (camshaft position sensor circuit), rough running especially at idle (loss of sequential injection on sensitive engines), VVT-related codes accompanying the CMP code (the PCM cannot control the phasers without cam position feedback), and occasionally a no-start if the engine requires cam signal to confirm the crank signal for initial start.

On engines with variable valve timing, a failing CMP sensor that produces an incorrect phase angle signal can cause the PCM to command the wrong VVT position, resulting in rough idle, poor fuel economy, and VVT-specific codes alongside the CMP circuit code. Always check what VVT codes are stored alongside a CMP code — they may all trace to the same sensor.

Cam/Crank Correlation Codes

Codes P0016 through P0019 indicate the PCM sees a misalignment between crank and cam position signals that exceeds the calibrated tolerance. The specific code identifies which bank and which camshaft (intake or exhaust) is out of correlation on multi-cam engines.

The most important thing to understand about correlation codes: they do not automatically mean the sensors are bad. The sensors are reporting what they see — and what they may be seeing is a real mechanical misalignment between crank and cam. The most common cause is a stretched timing chain on high-mileage engines. A chain that has stretched by even a few degrees of slack changes the relationship between crank and cam position beyond the PCM's tolerance threshold.

Before replacing any sensors on a correlation code: check timing chain condition. On overhead cam engines with known chain stretch issues (many GM 2.4L Ecotec, Honda K-series and J-series, Ford 3.5L EcoBoost), perform a timing chain inspection. Look for cam timing marks that are off when the crank is at TDC. Replacing sensors on an engine with a stretched timing chain fixes nothing — the new sensors will read the same misalignment and the code will return immediately.

Also consider VVT phaser issues. A stuck or failed phaser cannot move to the commanded position — the cam stays at a fixed angle relative to the crank regardless of PCM commands. If the default phaser position is outside the correlation tolerance window, a correlation code sets. Check VVT operation on the scan tool: command the phaser to maximum advance, watch whether actual cam angle changes to match the command. A phaser that does not respond to commands is the problem, not necessarily the CMP sensor.

Testing CKP and CMP Sensors

Hall effect sensor test: with the key on (engine off), check for 5V reference at the reference terminal. Check for solid ground at the ground terminal. Crank the engine and observe the signal wire — use a DVOM in AC mode (will show AC component of the switching signal), a scope, or a graphing multimeter. You should see a rapidly switching signal during cranking. No signal with correct power and ground: failed sensor. Incorrect signal pattern (flat, missing pulses): possible reluctor ring damage or air gap issue.

VR sensor test: test coil resistance with the sensor disconnected — compare to spec. Reconnect and crank while monitoring signal voltage on a scope or DVOM AC range. You should see an increasing AC sine wave as cranking speed builds. A flat scope trace during cranking with correct coil resistance suggests an air gap issue — check that the sensor is fully seated in its mounting bore. A flat scope trace with no measurable coil resistance (shorted winding) or infinite resistance (open winding): replace the sensor.

Reluctor ring inspection: with the sensor removed from its bore, rotate the crankshaft manually (socket on the crankshaft bolt) and observe the reluctor ring teeth through the sensor bore with a flashlight. Every tooth should be uniform, the reference gap should be consistent, and no teeth should be broken or damaged. Any damage visible through the bore requires further investigation — either remove the oil pan for better access or use an endoscope through the bore to inspect more thoroughly.

No-Start Diagnosis With Sensor Data

On a no-start with normal cranking speed, check for CKP and CMP codes first. P0335 (CKP circuit — no signal) or P0336 (CKP range/performance) stored alongside a no-start with a normal-sounding crank is a strong pointer to the CKP sensor or its circuit. Confirm with a scope: during cranking, you should see the CKP signal on the scan tool (look for RPM reading while cranking — if the scan tool shows zero RPM while the starter is engaged, the PCM has no CKP signal).

No-start without CKP codes does not rule out the sensor — some PCMs only store a CKP code after multiple failed start attempts or after the engine runs and then loses signal. If you have a no-start with no codes but the scan tool shows zero RPM during cranking (when the starter is clearly engaged and the engine is rotating), the CKP sensor circuit is your top suspect.

Frequently Asked Questions