P034B: Reported crankshaft direction reversals while the engine runs

P034B monitors whether the crankshaft is reported to be rotating in the wrong direction. The cited GM strategy counts crank-sensor reversals while the engine is running. It does not, by itself, prove the engine physically ran backward.

The qualifying reversal pattern

The published criterion is at least three reversal pulses within ten seconds. Evaluation occurs every 250 milliseconds, with engine speed above 400 and below 2,000 rpm, airflow at least 0.8 grams per second and detected engine movement.

Those boundaries distinguish this running-state diagnostic from arbitrary motion during stopping or an engine that is not moving. Direction information should be assessed in the operating window where the monitor made its judgment.

Validate direction data before mechanical conclusions

An active crank-sensor fault inhibits the test. A useful record combines reported direction changes, engine speed, airflow and crank-signal status. A speed display alone may not reveal the direction transitions the controller counted.

The source does not identify whether the cause lies in sensing, signal interpretation or actual motion. Use the correct directional crank-sensor and waveform procedure to resolve that distinction. Its Type B, two-trip classification also means a current normal event does not cancel stored evidence.

After diagnosis, confirm that the running monitor completes without qualifying reversals. Do not attempt to reproduce the code by deliberately forcing the engine to turn backward; that is not the documented test.

Source and applicability

This explanation is limited to the 2025 GM 25OBDG03A monitor on page 270. Its thresholds are calibration examples, not universal specifications. Confirm the vehicle, engine and controller procedure before applying them.