P0C85: Battery temperature sensor H varies too abruptly

P0C85 means hybrid/EV battery temperature sensor H circuit intermittent/erratic. The 2018 GM monitor treats repeated changes as evidence of instability; it is not a direct command to replace a battery-cooling component.

A short-window stability assessment

The diagnostic integrates absolute changes in the sensor signal over ten samples. The documented failure limit is above 10 for that accumulated value. Four failures out of five evaluations, sampled every 100 milliseconds, are required. GM lists Type B, two-trip reporting.

Two enable details affect interpretation: system voltage must exceed 10.2 V, and the prescribed battery-interface communication faults must not be active. Missing source data and erratic source data are different issues. If communication is invalid, this test may not establish anything about sensor H's stability.

Separate a possibility from a conclusion

The source describes an intermittent connection as a potential cause. It supplies no evidence that one particular connector, sensor or module is always responsible. Save the event trace and identify whether the instability was isolated to H or accompanied other reporting faults; that comparison informs the subsequent vehicle-specific investigation.

Confirmation after repair should show a stable H signal while the diagnostic is enabled, not only a code clear. Sensor letters must be mapped using the actual pack documentation. Internal battery electrical work remains a task for qualified high-voltage service, with no probing procedure implied by this explanation.

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 912.