P20C1: heater C maps to the second tank heater in this system

P20C1 is commonly labeled reductant heater C control circuit/open. The reviewed GM table calls the same monitored channel heater 3 and identifies it as tank heater 2. That numbering matters: it is not the line heater in this particular source.

The open diagnosis is completed off-line

The controller may detect an abnormal FET drain voltage or excessive load current while the heater is commanded on. It then turns the heater off because the specific circuit fault can only be pinpointed in that state. The open-circuit signature uses high-side ADC voltage between 0.926 and 1.083 volts together with zero low-side ADC voltage.

These are internal diagnostic measurements, not a connector probing procedure. A missing heater command after the initial event can therefore reflect deliberate fault isolation rather than the original cause of the open indication.

Verification requirements

Active vehicle power mode and absence of specified controller-internal faults are required. The failure qualification is 10 samples at 0.2 second each, described as two seconds. The table lists Type A, one-trip reporting and recovery only at the next driving cycle.

Confirmation should establish that the heater 3 off-state diagnostic no longer identifies an open path and that the subsequent eligible cycle completes. Testing another heater or observing tank temperature rise does not directly verify this channel. The source supplies no location, pinout or replacement verdict. Confirm the actual heater arrangement and applicable service information before assigning these 2025 GM facts to a vehicle.

Source: GM monitor, PDF page 197.