P0325: Knock-sensor continuity checked with an internal test signal

The cited GM P0325 monitor checks for an open in the Bank 1 knock-sensor circuit. It can use an internally generated 20 kHz signal or the engine's normal background noise, with the selected method depending on the calibration and engine speed.

Why the test signal behaves differently

In the 20 kHz method, the controller sends a signal through the knock-sensor circuit and analyzes the returned information. A sound circuit brings the signal to both inputs of a differential stage, producing a low diagnostic output. An open can leave it on only one input, producing a high output.

This makes a generic assumption that “more signal is better” incorrect for that method. The normal-noise alternative has different logic: adequate background activity supports circuit continuity. The source notes that some implementations change methods at high engine speed when the test signal separates good and bad circuits less clearly.

Know which method is active

Record engine speed and the applicable method definition before interpreting diagnostic output. The table requires more than two seconds of running and over 133 accumulated revolutions within its speed range, and filters the result using a 0.0100 weighting coefficient.

The document does not authorize injecting an external 20 kHz signal or striking engine components. Use the vehicle-specific circuit procedure. Verification should show a completed passing open-circuit evaluation in the appropriate method, rather than merely a visible knock-sensor waveform.

Source and applicability

This explanation is limited to the 2025 GM 25OBDG03A monitor on page 259, page 260, page 261. Its thresholds are calibration examples, not universal specifications. Confirm the vehicle, engine and controller procedure before applying them.