P0657: an actuator-supply open-load result saved at shutdown

P0657 is an open-load diagnostic for transmission high-side output 1 in the cited implementation. A distinctive feature is that the test runs at powerdown and the result is reported at the next startup.

The command sequence creates the evidence

With the high-side output disabled and its corresponding low-side output enabled, the load should let the low-side driver pull the high-side voltage down. If the HSO1 level remains above forty percent of battery voltage for at least 60 milliseconds, the open-load result is stored in nonvolatile memory.

The next startup reads that stored result and sets the DTC. A code appearing at startup can therefore describe the preceding shutdown test rather than a failure that first occurred during the new start.

Supply stability matters

The test requires battery voltage between 8.75 and 18 volts, with less than 1.5 volts of change during the test. P0882 and P0883 disable it. The forty-percent boundary follows battery voltage and should not be replaced with one fixed voltage for every test.

Verification should include a properly completed shutdown test followed by assessment of the next startup result. Observing only engine-running operation misses the diagnostic opportunity. The source identifies an open-load pattern, but does not locate the interrupted path or authorize probing an unidentified actuator circuit.

Documentation and applicability

These details describe the monitor in GM’s 2025 25OBDG07A Part2 file. Its application-matrix filename does not match the download index, so model and engine applicability remain unresolved. Use the vehicle’s correct service information before applying these calibration details.

GM diagnostic table, PDF page 936.