Diagnostic article Generic OBD-II
P20C2: Checking the second DEF tank heater’s resistance and power performance
Understand which heater P20C2 identifies and why electrical resistance and heating-power errors are separate tests.
Read before applying this article
Documented application
2025 GM 25OBDG04B DEFC monitor calibration only; this document does not establish applicability to every model, engine or reductant system.
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.
P20C2 is assigned to heater 3 in this GM system, described as the second tank heater. The numbering is easy to confuse: heater 2 is the line heater, while this monitor evaluates another tank-heating circuit.
Resistance and power tell different stories
The controller calculates resistance from voltage across the heater divided by current. For this calibration, values below 1.5 ohms or above 2.6 ohms are outside the listed resistance window and use an eight-second failure accumulation.
A second comparison checks delivered electrical power against commanded power. A difference greater than 29 watts in either direction is evaluated over ten seconds. A heater can therefore be investigated for excessive as well as insufficient power; the code is not limited to complete loss of heating.
Confirm that heater 3 was actually eligible
A positive PWM command is required. Faults elsewhere in the temperature, heater, supply or communication system may force the command to zero and stop the intended performance assessment. No recurrence during disabled heating is not evidence that the resistance and power comparisons passed.
Preserve the heater identity, command, measured current and the two sides of the voltage measurement used by the controller. These help distinguish a resistance-window failure from a power-tracking failure. The listed values describe a specific monitor calibration and must not be applied indiscriminately to another tank design. The source does not provide a safe direct-power test or identify the replacement component.
Source and scope
2025 GM 25OBDG04B DEFC monitor calibration only; this document does not establish applicability to every model, engine or reductant system. These are documented monitor criteria, not a universal repair procedure. GM PDF, physical page 197.
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 C Control Circuit Performance
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