P219A: Bank 1 imbalance inferred from oxygen-signal variation

P219A indicates bank 1 air-fuel imbalance in the reviewed GM strategy. The controller looks for unusually large variation in the pre-catalyst oxygen-sensor signal across cylinder events, rather than assigning the fault to a particular injector.

A statistical comparison, not an average mixture reading

Signal variance depends on speed and load. The monitor compares it with calibrated good-system behavior, applies normalization and a quality factor, then filters the result. The documented standard-mode failure boundary is a filtered ratio above 0.25. Following a prior-trip failure, the ratio must fall below minus 0.01 to report a pass, providing separation between failure and recovery.

A bank can therefore have an imbalance even when an average mixture reading seems plausible. Conversely, a noisy trace must be interpreted within the monitor's eligible operating region rather than judged by visual variation alone.

Stable operating conditions are required

The listed speed window is 1,100–4,000 rpm; idle operation is excluded. Short-term speed change must remain below 150 rpm, and airflow stability has separate limits. Data gathered during an abrupt transient may not count toward this diagnostic.

The front sensor is sampled once per cylinder event, so completing a sample takes different elapsed time at different speeds. The source also states that optional-mode imbalance calculation is not used in this application.

Verification should establish that the eligible imbalance test completes and its filtered result recovers. P219A identifies a bank-level pattern; cylinder attribution, mechanical checks and any injector repair require additional vehicle-specific evidence.

Source and applicability

The documented system and calibration must be confirmed for the vehicle before using these limits. GM diagnostic table, PDF page 610. GM diagnostic table, PDF page 611. GM diagnostic table, PDF page 612.