P22A0: NOx sensor 2 reports below its allowed concentration range

For the documented calibration, P22A0 means the post-catalyst NOx sensor's raw concentration has crossed the lower reporting limit. Bank 1 sensor 2 is the identification in the table. The failure boundary is below −90 ppm, so “circuit low” is implemented here through a concentration-range check, not a published connector-voltage threshold.

Conditions behind the code

The controller counts 100 failed observations out of 200 at a 25 ms task rate. It requires powertrain relay voltage above 11 V, sensor supply above 9.90 V, the sensor bus relay on and dewpoint reached. Communication and invalid-data faults must be absent, as must relevant electrical, current-control and oxygen-plausibility faults. Reporting is Type B over two trips.

How to interpret a negative value

The threshold is not a target exhaust concentration and does not mean that exhaust physically contains a negative quantity of NOx. It is an allowed signal-range boundary. A very negative raw reading directs attention toward measurement validity, while a low positive downstream reading alone does not satisfy this monitor.

For verification, determine whether the scan value is the raw sensor concentration and whether the sensor is qualified to run this test. A missing communication frame or a sensor still warming up is a different diagnostic state. The table supplies no justified replacement decision or universal harness test.

Source and application limits

This explanation covers the cited 2025 GM monitor. Confirm the vehicle and calibration in its service information before applying these limits; inclusion in this file does not establish model compatibility. GM diagnostic parameters, PDF page 1427.