P0391: Bank 2 exhaust-cam performance checks edge order and spacing

P0391 is the performance code for the bank 2 exhaust camshaft signal. In GM’s implementation the controller can reject a waveform that is electrically active but whose pattern fails to match the expected reference.

Presence of pulses is not enough

The listed failure alternatives include wrong segment length, a disturbed pattern that cannot match the reference table, an incorrect entry sequence, and too many erroneous edge positions. These checks concern pattern structure, rather than merely whether the input is high or low.

The defect counter is qualified by crankshaft revolutions. A crank gap signal must be detected, ignition must be on, and backward rotation must be absent. The reference conditions matter because cam-edge positions are judged relative to rotational context.

Use the correct waveform question

Retain a waveform long enough to show sequence and spacing, together with the crank reference. A snapshot proving that some pulses exist would not demonstrate that this pattern monitor has passed.

The strategy prose mentions implausible crankshaft operation, although its specific target and criteria identify the bank 2 exhaust-cam pattern. That wording is not used to diagnose the crank sensor by assumption. The source supports a cam-pattern investigation with one-trip reporting; it does not establish the failed sensor, harness or timing component. The appropriate engine reference pattern and service procedure are necessary to localize the cause.

Source and applicability

This explanation covers the cited GM monitor, not every vehicle carrying this code. The application-matrix filename does not resolve cleanly to this file, so no model or engine fitment is assigned. GM diagnostic table, PDF page 593; GM diagnostic table, PDF page 594.