P0C91: Sensor J's intermittent fault differs from its range faults

P0C91 identifies intermittent or erratic hybrid/EV battery temperature sensor J behavior. The 2018 GM table gives it a different test from J's range/performance and circuit-endpoint codes: it measures accumulated changes in the signal.

The pattern matters more than the endpoint

Absolute signal movement is integrated over ten samples. A result above 10 fails the listed criterion, with four failing evaluations out of five at 100-millisecond intervals required. A jump away from a plausible value and back can still contribute strongly even though the final reading appears normal.

The diagnostic requires system voltage above 10.2 V and no active battery-interface communication faults from the defined group. Its classification is Type B, two trips. The source discusses intermittent connection behavior as one possible explanation, not a confirmed part diagnosis.

Interpreting several sensor J codes

If J also has an endpoint or plausibility code, preserve their status and timing. A current unstable trace and a historical out-of-range event provide different pieces of evidence; do not flatten them into a single “bad temperature” conclusion. Use the applicable service sequence to decide which current issue to resolve first.

Verify stable J data during a completed eligible monitor run after repair. Locate the actual sensor through vehicle-specific documentation. Internal pack diagnosis requires qualified high-voltage service; this article does not supply a direct electrical test or disassembly method.

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 920.