Diagnostic article Generic OBD-II
P248B: Reductant heater A supply high relative to the powertrain relay
P248B compares DEF tank-heater supply with relay voltage; see the documented difference threshold, heater command and sample requirements.
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.
The GM P248B row checks excessive DEF tank-heater supply voltage relative to the powertrain relay. This is a differential electrical check. A high battery-voltage reading on its own does not establish the condition, and a low tank temperature does not demonstrate this particular fault.
The reference voltage determines the meaning
The failure criterion is tank-heater supply voltage minus powertrain-relay voltage greater than 2.0 V. Preserve both readings under the same conditions: substituting battery voltage for the named relay reference can change the interpretation. The tank heater must be commanded on for this monitor.
The table requires run/crank voltage above 11.0 V and powertrain-relay voltage above 11.0 V. It additionally lists run/crank voltage above 6.0 V or engine running, excludes engine cranking, and requires no U010E reductant-module communication fault. These are stated prerequisites, not instructions to force a heater on or bypass its control.
Qualification and next diagnostic evidence
Sixteen failures out of 20 samples are needed, with each sample taken at 500 ms. Type A, one-trip reporting is specified. A brief supply transition should be distinguished from the sustained differential captured by that count.
Record the two supply channels, command state and communication status before clearing the code. If the channels disagree, identify where each value is measured or reported in the correct wiring and controller documentation. The table does not locate a short or prove a failed heater element. A common system-voltage rise and a tank-heater supply that rises above its relay reference are different electrical patterns; the latter is the comparison relevant to this monitor.
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 401.
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 A Sense Circuit High
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