P0C99: Sensor L circuit low produces a high-temperature conversion

P0C99 concerns hybrid/EV battery temperature sensor L circuit low. The documented GM circuit uses an NTC thermistor with a pull-up on the sensing board. High temperature and a sensor short to ground both move the voltage in the low direction.

Understanding the converted threshold

The 2018 table compares calculated temperature with a limit above 160 °C. Eight failures among ten samples at 100-millisecond intervals meet the monitor's requirement. System voltage must be above 10.2 V, and the specified battery-interface communication faults must be absent. The classification is Type B, two trips.

Because this is an electrical endpoint monitor, an extreme displayed temperature is not by itself proof of the battery's actual temperature. The threshold also must not be used as an operating or safe-driving limit.

Preserve enough context to separate causes

Retain L's reported temperature and any available signal data with the other temperature measurements and active warnings. The vehicle-specific procedure must distinguish real thermal conditions from the circuit behavior GM describes; the DTC cannot make that distinction alone.

A verified correction should produce credible L data during a completed eligible circuit check. Merely removing a stored code does not establish this. Use the exact vehicle's location and circuit information before inspection. Pack access and internal sensor testing require qualified high-voltage service, without improvised probing or energization based on this summary.

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 926.