P220B: post-catalyst NOx sensor supply-voltage supervision

P220B identifies the supply-voltage monitor for bank 1 NOx sensor 2. The reviewed GM row calls this the post-catalyst sensor. Its failure criterion is sensor supply below 9.90 volts, rather than a low NOx reading or poor catalyst conversion.

Keep sensor 2 separate from the upstream channel

The table uses the sensor B communication and data-validity paths for this check. A normal supply reading or clean communication result from the engine-out sensor does not independently establish that the post-catalyst unit is healthy. Channel identity matters when both sensors appear on the same scan report.

The powertrain-relay supply must exceed 11 volts and the NOx bus relay must be commanded on. The engine must be running and the sensor's dewpoint condition reached. Lost communication and invalid CAN-data faults must be absent, so unavailable data is not treated as a confirmed low-supply measurement.

Confirming an eligible result

Failure qualification uses 125 samples out of 250 at 25-millisecond intervals. Reporting is Type B over two trips. The sample counter is separate from the time needed to reach the enabling conditions.

Verification should establish valid sensor 2 communication and an executed supply check with the relay conditions satisfied. A catalyst-efficiency result or plausible downstream concentration does not substitute for that electrical evidence. The source provides no circuit location or replacement instruction. Confirm the installed post-catalyst sensor arrangement and vehicle-specific service procedure before applying this 2025 GM calibration.

Source: GM monitor, PDF page 1416.