P22A4: NOx sensor 2 heater-control short to ground

P22A4 identifies low-voltage behavior in NOx sensor 2's heater-control circuit. The GM row describes this as a short to ground on the post-catalyst sensor's heater-control pin. It does not describe a low exhaust NOx reading or a low exhaust temperature.

When the fault is evaluated

This check requires the NOx bus relay on, powertrain relay voltage above 11 V and sensor supply above 9.90 V. It also requires the sensor to have reached dewpoint, with neither lost communication nor invalid-data failure. The counter is 80 failures out of 160 samples, evaluated at 25 ms intervals. Reporting is Type B, two trips.

The dewpoint condition separates the test's eligibility from its electrical failure criterion. A cold-start scan with no new failure may simply precede monitor activation. Likewise, a communication fault can prevent the controller from accepting the sensor's electrical report.

Useful next evidence

Identify the heater-control circuit in the correct service diagram rather than treating every NOx-sensor wire as interchangeable. The adjacent heater-sense codes concern another measurement path. Review the heater-control fault status after the supply, communication and dewpoint requirements are met. The summary table does not publish a measured voltage cutoff or a safe pin-probing method, so those details must come from the exact vehicle procedure.

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 1431.