P0D09: Charger voltage stays low during charger precharge

P0D09 identifies battery charging system positive contactor A stuck open. The 2018 GM HPC2/VICM monitor reaches this result by checking charger voltage during charger precharge, rather than by directly observing the power contacts.

The reference bus must already have voltage

The failure threshold is charger voltage below 10 V. The row requires the charger and precharge contactor states to be closed, the propulsion negative contactor open, valid charger and propulsion negative-bus voltage measurements, and negative-bus voltage above 40 V.

Eight failed samples out of nine at 12.5 milliseconds per sample meet the 112.5-millisecond evaluation. It executes once per charger precharge and has Type A, one-trip reporting.

Why the conditions affect the conclusion

A low charger voltage with no established reference bus or outside the precharge sequence is not equivalent to this fault. Preserve the contactor states, the two voltage measurements and their validity flags. They establish whether the diagnostic was judging the intended charging path under meaningful conditions.

The code does not isolate a mechanical contactor defect from every other reason voltage failed to reach the monitored point. Follow the vehicle-specific diagnostic before choosing a component. No direct energization, contactor bridging or high-voltage measurement procedure is supplied here. Qualified high-voltage service must verify the appropriate charging sequence and monitor result after the underlying issue is corrected.

Source and applicability

This explanation follows the documented GM monitor, not a verified application to every vehicle with this code. Confirm the vehicle, reporting module and calibration in current service information. GM diagnostic table, physical page 989.