P20C0: A DEF line-heater short-to-power report and its diagnostic shutdown

P20C0 applies to the line heater, numbered heater 2 in this GM document. The monitor reports an electrical short-to-power condition in its controlled circuit. The “high” designation describes the circuit failure category, not the tank temperature or the amount of reductant in the system.

The commanded state changes during detection

When an abnormality is found while heating is requested, the controller switches the output off. This is necessary because the source's specific fault classification depends on the off-state high-side and low-side ADC pattern.

The presence of voltage in a deactivated circuit is therefore interpreted within a defined internal test, not by assuming that every nonzero reading at an external connector is a short. This summary supplies no connector pinout or meter procedure for that inference.

Distinguish the initial problem from its consequence

A stopped heating command can follow the electrical failure. It should not be mistaken for evidence that the controller was simply unwilling to heat the line. The documented confirmation is sample-based over two seconds, and recovery is assigned to the following driving cycle.

Useful event information includes which line-heater diagnostic was reported, whether the output had been on and what classification followed shutdown. These observations separate P20C0 from an open circuit or a low-circuit ground-short report. The component responsible still has to be isolated with the applicable vehicle circuit information; the code does not justify a tank assembly or heater replacement on its own.

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 196.