P20C4: heater 3 short-to-power is confirmed with heating switched off

P20C4 is the circuit-high code for reductant heater 3 in the reviewed GM controller. The table identifies that channel as tank heater 2, corresponding to the dictionary's heater C label. The diagnostic evaluates whether the high- or low-side control circuit is shorted toward power.

Shutdown helps identify the fault

An abnormal on-state drain-voltage or current indication can cause the heater to be turned off. The source explains that the precise control-circuit failure is then pinpointed in the off state. Its high-circuit signature combines high-side and low-side ADC voltage windows, rather than relying on a high current reading alone.

The documented off-state windows are approximately 2.021–4.168 volts for the high-side ADC and 2.097–4.325 volts for the low-side ADC. These values belong to the controller's diagnostic measurement path; they do not establish external connector pin test limits without the applicable circuit information.

Interpreting a retest

The vehicle must have an active power mode and no specified controller-internal fault. Two seconds, represented by 10 samples at 0.2 second each, qualifies the diagnosis. Type A, one-trip reporting and next-driving-cycle recovery are listed.

Verification should follow the off-state classification and later eligible-cycle result. A heater that remains off immediately after detection is not independent proof of a failed controller. The page gives no root-cause ranking or replacement instruction. Exact vehicle applicability and safe circuit isolation require the proper heater-system service procedure.

Source: GM monitor, PDF page 198.