P248D: A DEF heater sense-high result is a voltage comparison

P248D concerns the diesel exhaust fluid line-heater supply being too high relative to its reference supply. In this GM monitor, the important quantity is the difference between line-heater voltage and powertrain-relay voltage. A high reading should therefore be interpreted with both values, rather than as a stand-alone battery-voltage fault.

Compare the supplies under the recorded command

The failure threshold is a difference above 2 V. The check requires appropriate run/crank and relay supplies, an engine that is not cranking, and a commanded-on tank heater. The row specifically names the tank-heater command even though the monitored supply is for the line heater. Replacing that enable condition with a guessed line-heater command would change the documented test.

The monitor also excludes U010E. Communication availability consequently affects whether this heater diagnosis is eligible; an absent heater result during a communication fault does not establish normal electrical operation.

Distinguish an event from its confirmation

The calibration requires 16 failed observations among 20 samples, with 500 ms between samples. One isolated voltage spike is not the same evidence as the completed sample decision. Preserve the supply difference and operating state across the relevant interval.

This finding concerns electrical supply supervision. It does not measure fluid temperature, prove a blocked DEF line, or identify which supply path caused the difference. Circuit localization needs the applicable wiring and controller procedure.

Source and applicability

2025 GM 25OBDG06C documented monitor only. Vehicle, engine and calibration applicability must be confirmed from the matching service information; no model fitment is inferred. These criteria explain controller logic, not a complete repair procedure. 25OBDG06C, PDF page 403.