P248A: Reductant heater A supply low in the GM tank-heater monitor

P248A is described here as DEF tank-heater supply undervoltage monitoring. Although a generic code label may call it heater A sense circuit low, this document connects the monitored quantity to the tank heater's supply. It is not a direct measurement of DEF temperature or heating effectiveness.

Compare two voltages under a valid request

The failure calculation subtracts tank-heater supply voltage from system battery voltage. It fails when that difference exceeds the printed 3.00 threshold. The table omits a unit beside that number; the voltage comparison is clear, but confirm the detailed service limit before applying it as a measurement specification.

The heater must be requested on by the heating strategy. Calibration enable, valid powertrain-relay supply, active run/crank and exclusion of cranking are also required. Lost communication with the reductant control module, identified as U010E, must be absent. A heater that is intentionally off does not recreate the documented test state.

Data that separates the likely paths

Capture battery voltage, reported tank-heater supply, heater request and reductant-module communication together. The important evidence is their difference during the same operating condition, rather than a heater-voltage reading from one moment compared with battery voltage from another.

The monitor requires 16 failures among 20 samples with a 500 ms time basis and lists Type A, one-trip reporting. Use that qualification when distinguishing sustained loss from a brief transition. Check the supply path using the vehicle's wiring and test procedure; this row alone cannot determine whether a connection, module or heater is responsible, and it does not establish that the tank assembly needs replacement.

Source and applicability

2025 GM 06C documented monitor only. Vehicle, engine and controller calibration applicability must be established separately; no model fitment is inferred from the file. GM PDF, physical page 400.