Camshaft Position Sensor

The camshaft position sensor (CMP sensor) is the secondary engine position reference sensor that works in conjunction with the crankshaft position sensor to provide the ECU with cylinder identification — the ability to determine which cylinder is on its compression stroke (ready to fire) rather than its exhaust stroke (which also has the piston near TDC). While the crankshaft position sensor provides engine speed and position data referenced to TDC, it cannot alone distinguish between the two TDC events per 720 degrees of four-stroke cycle (one compression TDC where the cylinder fires, and one exhaust TDC where it does not). The camshaft position sensor, reading from a reluctor ring or single triggering lobe on the camshaft, provides a once-per-720-degree reference pulse that allows the ECU to identify cylinder 1’s compression TDC specifically, enabling it to sequence injection and ignition for each cylinder in the correct firing order. On DOHC engines with variable valve timing, there is typically one CMP sensor per camshaft — both intake and exhaust — with the sensors providing position feedback for the VVT phaser control loops in addition to their synchronisation role. Without a functioning CMP sensor, the ECU cannot sequence individual injector firing and ignition coil activation, forcing it to either cut fuel injection entirely or to use a default symmetric pattern that is less fuel-efficient and emissions-compliant.

The CMP sensor’s single pulse per cycle is sufficient for the ECU to establish the firing order reference because the crankshaft position sensor provides high-resolution position data continuously — the ECU only needs the camshaft pulse to synchronise once, after which it tracks cylinder position continuously from the CKP signal. After an engine start, the ECU uses the first CMP pulse it receives to establish which cylinder is at compression TDC and begins sequential injection immediately. On some engines, if the CMP sensor fails after the engine has already started and synchronisation has been established, the engine continues running using only the CKP signal and the previously established synchronisation reference — but will not be able to re-synchronise after a shutdown and restart. On turbocharged engines with VVT, the CMP sensors on both intake and exhaust camshafts provide continuous feedback on phaser position, allowing the ECU to command and verify phaser movements in real time. A failed CMP sensor on a VVT phaser circuit disables variable valve timing on that camshaft, locking it at its default (spring-loaded) position and reducing power and efficiency across the operating range.

Engine cranks but does not start: A completely failed CMP sensor prevents the ECU from establishing cylinder synchronisation at start-up. The engine cranks normally (the starter and crankshaft are functioning) but never fires — the ECU cannot determine which cylinder to inject fuel into first. The symptom is distinguishable from a CKP sensor failure because the engine may still briefly fire on a few random events before dying, as the ECU attempts injection on estimated timing without synchronisation confirmation.

VVT system fault with reduced performance: A failing CMP sensor on a DOHC VVT engine generates erratic position data for the phaser control loop, causing the ECU to detect a phaser position error and freeze the VVT system at the default timing position. The check engine light illuminates with a VVT or camshaft position correlation fault code. The engine runs but loses its variable timing authority, reducing low-RPM torque and high-RPM power compared to normal operation.

Intermittent stall or hard start in hot conditions: Like the CKP sensor, CMP sensor failures are often heat-sensitive — the sensor’s internal wiring or the connector corrosion causes an open circuit when thermally expanded. Hot restarts fail while cold starts succeed. This pattern, combined with a CMP fault code, is the diagnostic indicator for heat-sensitive sensor failure.

Level 1 — Starting Pattern Monitoring: Note any pattern where the engine starts readily when cold but refuses to start or starts very poorly when hot (immediately after a short stop, or after leaving a hot engine for 30–60 minutes). This ‘hot-soak’ no-start pattern is one of the most consistent indicators of a heat-sensitive position sensor failure — either CKP or CMP. Reading fault codes during the hot-start failure event will typically identify which sensor is at fault. Also monitor for any loss of power or increased fuel consumption that appeared at the same time as a check engine light, which may indicate VVT disable from a CMP fault.

Level 2 — Connector Inspection at Cam Cover Service: The CMP sensor connector on most engines is accessible when the valve cover is removed for gasket or spark plug service. Inspect the connector and the sensor body for oil contamination — a sensor immersed in leaked oil from a failing cam seal degrades faster than normal. Clean and re-seat the connector and verify the sensor is correctly retained in its mounting hole without play.

Level 3 — Professional Replacement: CMP sensor replacement is typically straightforward — one retaining bolt, one connector. However, on some engines the sensor is positioned behind the timing cover or requires partial cam cover removal for access. After replacement, clear all fault codes and confirm VVT operation (if applicable) by performing a short drive cycle that includes conditions where VVT advance and retard are commanded — the VVT system’s correct operation is confirmed by checking for absence of cam timing fault codes after the drive cycle.