P221F: second tank-heater channel draws excessive current

P221F is the current-too-high code for reductant heater 3 in the reviewed GM source. The same row names it tank heater 2; the generic dictionary calls it heater C. Understanding that mapping prevents applying the diagnosis to the line heater or the first tank-heater channel.

The monitor judges sustained current

Current above 15 amperes must accumulate over 40 samples at a 0.1-second time base, giving a four-second failure requirement. This electrical limit is not a direct judgment of tank-fluid temperature and does not establish that the heater's thermal output is too high.

Other reductant-system faults can set heater commands to zero. The source lists temperature-sensing, other-heater, power-supply, communication and Run/Crank influences. The command may therefore disappear after detection or because of another fault, making the original command/current record more useful than a later static zero-current reading.

Verification and scope

The vehicle must be in an active power mode, with specified controller-internal faults absent. Type A reporting over one trip and recovery only at the next driving cycle are listed.

Confirmation should follow the third heater's current and diagnostic status through an eligible later cycle. A clean heater-control short test is separate evidence and does not by itself establish that the operating current remains within its limit. The summary identifies no failed component or repair instruction. Confirm the actual heater arrangement and use vehicle-specific service information for testing and any replacement decision.

Source: GM monitor, PDF page 205.