P220A: NOx sensor 1 supply falls low while relay voltage remains adequate

P220A is the supply-voltage code for bank 1 NOx sensor 1, identified here as the engine-out sensor. The reviewed GM monitor detects sensor supply below 9.90 volts. It separately requires the powertrain-relay voltage to be above 11 volts.

The two voltages answer different questions

The relay-supply prerequisite qualifies the broader powered environment; the sensor-supply criterion identifies the low condition being judged. A generic battery reading alone cannot establish either the sensor's reported supply status or the entire enabled diagnostic result.

The NOx bus relay must be commanded on, the engine running and dewpoint reached. Lost communication and invalid sensor CAN data must be absent. Thus a missing sensor message and a valid report of low sensor supply are distinct findings, even if both can affect the available NOx data.

Qualification and verification

The source specifies 125 failing samples within 250 at a 25-millisecond task rate, with Type B reporting across two trips. A momentary low reading is not automatically the full qualified condition, and the code is not a direct NOx concentration or catalyst-efficiency diagnosis.

Verification should compare the sensor 1 supply status with relay voltage and relay command during an eligible engine-running period. No code while communication is lost does not establish a supply pass. The table identifies no fuse, ground point, connector pin or replacement component. These facts apply only to the documented 2025 GM monitor until the actual engine-out sensor system and vehicle service information are confirmed.

Source: GM monitor, PDF page 1415.