P221C: Why low DEF line-heater current must be judged with a positive command

P221C is a low-current monitor for heater 2, the reductant line heater in this GM system. It asks whether the energized circuit draws enough current for the requested operation. A heater that has not been commanded on is outside that specific comparison.

Demand separates an idle load from a fault

The row requires line-heater PWM greater than zero and sets the low-current boundary below 0.75 A. Failure accumulation is four seconds, represented by forty samples taken every 0.1 second.

Those two facts must be considered together. Zero current with zero command does not reproduce the failure criterion, while an apparently active command with persistently insufficient current is relevant evidence. The monitor does not simply declare every cold line to be an electrical fault.

Other faults may remove the test conditions

The table lists temperature, heater, power-supply and communication faults that can force heating commands to zero. After such a shutdown, absence of another P221C result cannot demonstrate that the line-heater load is healthy. The document also places recovery on the following driving cycle.

Record command and current at the same time, along with the reason heating was enabled or disabled. This allows the low-current event to be separated from an intentionally inactive circuit. The monitor narrows the investigation to insufficient electrical load under demand, but does not identify an open connector, a heater element or another specific cause without vehicle-level circuit testing.

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 203.