P221E: Identifying inadequate current in the second DEF tank heater

P221E refers to heater 3 in the cited GM table, described there as the second tank heater. It is a low-current diagnosis. Confusing that target with heater 2 would direct the investigation toward the line heater instead of the circuit named by this monitor.

The current needs a matching heating request

The listed failure condition is current below 0.75 A while the second tank heater's PWM request is above zero. The controller accumulates forty failing samples at 0.1-second intervals. Insufficient current has to be observed under demand; an inactive heater normally provides no useful evidence for this comparison.

This is not the same as a resistance or power-tracking performance test. Those compare additional electrical quantities, whereas P221E focuses on minimum current in the requested heater circuit.

Verify why heating stopped

Several other system faults can command PWM to zero, including temperature, heater supply and communication problems. A current reading taken after that inhibition may explain nothing about the earlier loaded state. The table describes recovery on the next driving cycle, so a short post-fault observation is not a complete verification.

Preserve the correct heater identifier, command, current and inhibition status. These observations establish whether a real low-load event occurred. The source does not provide a tank disassembly procedure or uniquely identify the heating element as failed; the component and wiring cause still need to be isolated using the applicable service information.

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 204.