P229F: post-catalyst NOx sensor current-control instability

P229F is a range/performance code for bank 1 sensor 2, but the cited GM strategy is unusually specific: it evaluates the post-catalyst NOx sensor's embedded current-control stability. It does not simply compare exhaust NOx concentration against an emissions limit.

The monitored evidence

The sensor supplies stability information for both NOx and lambda control. The table relates those flags to internal voltage and pump-current behavior, then checks the fraction of time the stability flag is off. Both paths show an off-time fraction greater than 0.50% as the malfunction threshold. Reporting uses two failed samples out of two on a 12.5 ms task and is designated Type B, two trips.

These short counters apply only after qualification. The sensor must have reached dewpoint, supply must exceed 9.90 V, and heater resistance must remain close to its target for more than ten seconds. Communication, electrical and oxygen-plausibility faults also matter.

What changes the diagnosis

A high NOx concentration and an unstable internal control loop are different observations. Preserve the sensor's stability or status data where the vehicle supports it, alongside its concentration reading. Also establish heater readiness before judging a replacement or repair: an unqualified cold sensor has not demonstrated a passing stability test. The source describes the monitor, not a component-replacement verdict.

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 1425. GM diagnostic parameters, PDF page 1426.