P20C3: Locating the meaning of a low circuit on the second DEF tank heater

For the GM monitor cited here, P20C3 refers to heater 3, the second DEF tank heater. Its electrical control circuit is diagnosed as shorted to ground. This identity distinguishes the target from both the first tank heater and the separate line heater.

The off-state pattern provides the classification

The monitor may initially see a fault while the heater is on, but it switches the command off before pinpointing the circuit failure. The low-circuit classification uses its own high-side and low-side ADC pattern in that off state.

A zero command after detection is consequently expected behavior in this sequence. It does not demonstrate an unrelated lack of heating demand. The listed failure period is two seconds, accumulated over ten samples, with recovery assigned to the next driving cycle.

What follows from a ground-short category

The code directs attention to the electrical path for this particular heater; it does not establish that the fluid itself is abnormally cold or that the heating element is necessarily the failed component. Both controlled sides of the circuit are included in the source description.

The useful next evidence is the actual circuit topology and a vehicle-specific test that can separate a ground path from an open or a short to supply. Record the original driver category before clearing anything, together with the heater command history. The internal diagnostic thresholds in a summary table are not a substitute for connector-level service instructions or a justification to energize the circuit manually.

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.