P23AD: NOx sensor 3 internal current control is unstable

P23AD concerns NOx sensor 3, described here as the sensor after the second catalyst. Its range/performance test monitors internal NOx and lambda current-control stability. A concentration reading by itself is not the complete diagnostic evidence.

The stability calculation

The controller evaluates the fraction of monitored time that each stability flag remains off. The printed threshold is greater than 0.50%, with two failures out of two samples on a 12.5 ms task and Type B, two-trip reporting. The sensor's internal voltage and pump-current criteria produce those flags.

The test requires dewpoint, suitable supply, valid communication and a heater-resistance relationship sustained for more than ten seconds. Fuel-request stability also qualifies the test. These prerequisites explain why rapid reporting after enablement can coexist with a longer delay before the monitor begins.

What deserves attention

Examine stability status along with sensor concentration and heater readiness. A transient concentration change does not necessarily mean the internal current controller is unstable, while repeated unstable flags deserve attention even when a displayed average seems plausible. The source has an isolated “NOx2” electrical-failure label beside a sensor-3 signal name; it should not redirect diagnosis to sensor 2. Confirm sensor identity and the applicable calibration before acting on that inconsistent wording.

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 1452.