P0D18: Charging interlock high in two different source states

Two routes to an interlock-high result

P0D18 in the cited GM table is not a single fixed-voltage test. It monitors charging high-voltage-interlock feedback while the source is both off and on. In the unsourced state, identified as 0 volts, feedback above 24% of reference meets one failure criterion. In the sourced 5-volt state, feedback above 44% meets the other.

Both branches require the 12-volt battery above 10.2 volts. A high result with the source off is therefore interpreted differently from an excessive response with the source on. The measured percentage and commanded source state must be kept together when reviewing diagnostic data.

The branches also use different counters

The unsourced test needs five failures in eight samples. The sourced test needs four failures in six samples. Both sample every 12.5 milliseconds, and the table lists a 75-millisecond passing duration. Combining the thresholds into one generic “high-voltage reading” rule would lose both the state distinction and the monitoring logic.

For diagnosis, establish which branch was active and whether the supply prerequisite was satisfied. Compare companion interlock codes with that sequence rather than assuming every interlock fault means an open connector. This table identifies an unexpected electrical response; it does not locate the failed part or certify the high-voltage system’s isolation.

The timing values are monitor criteria, not discharge times for safe service. Charging-system access requires the prescribed isolation and verification procedure. Never bridge an interlock circuit or apply an external voltage merely to force a passing state.

Source and applicability

15OBDG01 Hybrid Diagnostics, cited controller and monitor only. Historical calibration limits are not universal specifications; per-code vehicle, engine and hardware applicability must be verified.

GM source, physical page 555.