P221D: reductant line-heater current exceeds its operating limit

P221D is a high-current diagnosis for reductant heater 2. The reviewed GM source identifies that channel as the line heater, corresponding to the dictionary's heater B designation. This is a current-limit monitor, separate from heater resistance or power-tracking performance.

Current and command history matter

The failure threshold is greater than 15 amperes. Qualification uses 40 samples at 0.1 second each, or four seconds. The source does not say that any single current peak is enough to set this code, nor does it identify a physical short location from the threshold alone.

The table also explains that several temperature, heater, supply and Run/Crank faults can force heater PWM commands to zero. A heater observed off after a fault may therefore have been disabled by the controller. That later zero-current state does not disprove the earlier overcurrent event.

Confirming the electrical outcome

An active vehicle power mode is required and specified reductant-controller internal faults must be absent. Reporting is Type A over one trip, with recovery only at the next driving cycle.

Verification should evaluate heater 2's current and command history during an eligible test and confirm the appropriate next-cycle result. A successful test of either tank heater does not validate the line-heater channel. The monitor provides no basis for choosing a replacement harness, heater or module without further diagnosis. Exact heater mapping, safe electrical access and vehicle applicability require the service procedure for the installed 2025 GM reductant system.

Source: GM monitor, PDF page 204.