Diagnostic article Generic OBD-II
P248C: Reductant heater B supply low in the GM line-heater monitor
P248C monitors a DEF line-heater supply deficit while heating is requested, with specific sample and communication conditions.
Read before applying this article
Documented application
2025 GM 06C documented monitor only. Vehicle, engine and controller calibration applicability must be established separately; no model fitment is inferred from the file.
The code alone does not identify a failed part or establish that a vehicle is safe to drive. Confirm the vehicle, reporting module and applicable service procedure.
This GM source describes P248C as DEF line-heater supply undervoltage. The monitored heater serves the line, so a generic heater B label should not automatically direct diagnosis to the tank heater. Confirm the vehicle's heater-channel mapping before selecting connectors or parts.
Look for a supply deficit during heating
The calculation is system battery voltage minus line-heater supply voltage. A result above the printed 3.00 threshold is the failure condition. No unit accompanies that threshold in the original table; use the relationship to interpret the monitor, and obtain the applicable service specification before making a pass/fail electrical measurement.
A heating request is essential. The row also requires calibration enable, powertrain-relay supply in range, active run/crank and exclusion of cranking. Communication with the reductant control module must be present; U010E is identified as the communication fault that prevents the stated condition.
Keep the line and tank observations separate
Record battery voltage and line-heater supply at the same time as the line-heater command. A tank-heater supply reading cannot stand in for the line channel, even when both heaters share parts of the upstream supply. Correlating both channels can help establish whether the observed deficit is confined to one monitored branch, but this table does not supply their wiring topology.
The decision requires 16 failures in 20 samples at a 500 ms time basis, with Type A, one-trip behavior. Verify whether the fault returns during an eligible heating request. A cold line or an inactive heater alone is insufficient evidence for this electrical code, and the monitor cannot identify the failed component without circuit-specific testing.
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 402.
Article source documents
Source checks describe how the text was prepared; they do not certify a diagnosis or confirm compatibility with every vehicle using this code.
Original code reference
Dictionary definition
Reductant Heater B Sense Circuit Low
This definition retains its original catalog scope. The article above is limited to its explicitly documented application.
Reference scope
Generic OBD-II definition.
A manufacturer label does not establish the model, year, engine or module to which a diagnosis applies. Read the safety and diagnostic limits.
Definition source
- DTC Database: pinned community code definitions
Source checked · MIT