Crank / Cam Correlation: Why Both Signals Can Look Fine But Correlate Wrong

What this guide covers

Scoping crank vs cam correlation — the phase relationship between the crankshaft and camshaft signals that the ECM uses for sequential injection and ignition timing. Why both signals can look individually plausible but correlate incorrectly, and how that points to mechanical timing faults. Pairs with P0335, P0340, and correlation codes P0016/P0017.

Synchronisation concept

The ECM identifies which stroke each cylinder is on (intake, compression, power, exhaust) using the fixed phase relationship between:

• The crankshaft signal (with its missing-tooth reference for TDC)
• The camshaft signal (identifying which half of the 4-stroke cycle)

Without both, the ECM cannot do sequential injection — it falls back to batch fire. Without crank, it cannot fire at all. The correlation between the two is the synchronisation the ECM relies on.

Timing relationship

On a dual-trace scope (crank on channel 1, cam on channel 2), the cam pulse must occur at the correct phase relative to the crank missing-tooth reference. That phase is fixed by the mechanical timing (chain/belt, phaser position). The scope shows whether the cam pulse lands where it should in the crank cycle.

Mechanical timing faults

When the chain or belt jumps, or a VVT/cam phaser is mistimed, the cam signal phase-shifts relative to the crank. Both signals are individually clean — the sensors work — but the correlation is wrong. The ECM sets P0016/P0017 (correlation) alongside or instead of P0335/P0340. The scope confirms the phase shift; the scan tool shows the correlation code.

Sensor dropout

If either signal drops out intermittently, the ECM loses correlation intermittently — long cranks, intermittent no-starts, batch-fire fallback. The scope catches the dropout that the scan tool reports as an intermittent code. See the Crank Sensor Waveform guide.

Trigger-wheel faults

A damaged or loose trigger wheel (reluctor) on either the crank or cam produces a distorted pattern — a missing, extra, or shifted pulse. The scope shows the physical pattern error; the ECM sees a correlation fault. This is not a sensor failure; it is mechanical.

Timing-chain / belt shift concept

A jumped chain or stretched belt shifts the cam phase. Symptoms: cam/cam correlation codes, long crank, rough idle, sometimes piston-to-valve contact on interference engines. The dual-trace scope is the decisive test — it shows the phase shift that confirms mechanical timing, before any cover comes off.

Why both signals can look individually plausible but correlate incorrectly

This is the key insight: each signal is clean — correct amplitude, clean edges, correct frequency — but the phase relationship between them is wrong. A single-channel scope of either sensor looks fine; only the dual-trace correlation reveals the fault. This is why correlation codes (P0016/P0017) cannot be diagnosed by looking at either sensor alone.

Relationship to P0335 / P0340

• P0335 / P0340 — crank / cam signal faults (sensor or wiring)
• P0016 / P0017 — crank/cam correlation faults (mechanical timing or phaser)

The dual-trace scope separates them: a clean-but-phase-shifted pattern → mechanical timing; a corrupt/missing pattern → sensor/wiring.

Specification warning

The correct cam-to-crank phase relationship is specific to each engine. Vehicle-specific verified specification required. Compare to a known-good waveform for that engine; do not assume a universal phase.

Common mistakes

• Scoping only one channel (misses the correlation)
• Condemning a sensor when the correlation is the fault
• Not checking VVT/cam-phaser command vs actual on variable-timing engines
• Ignoring correlation codes (P0016/P0017) that point to mechanical timing
• Replacing the cam sensor when the chain has jumped
• Cranking repeatedly on an interference engine with a suspected jumped chain (piston-to-valve risk)