P21F8: Why bank 1’s lean idle code depends on the port-injection adaptation

P21F8 describes fuel-control system B too lean at idle on bank 1. The GM table ties this to the maximum additive mixture adaptation on the port-fuel-injection path. It should not be treated as an undifferentiated fuel-trim fault across every available injection mode.

A specific correction crosses a specific boundary

The source gives an upper additive value of 5.8125. That internal correction scale is not labeled as a conventional short-term fuel-trim percentage. Identifying the scan-tool parameter and representation is necessary before deciding whether a displayed value matches the diagnostic input.

The additive PFI integrator must be stable after the multiplicative PFI integrator. The following page requires pure port injection to be active for more than two seconds and qualified lambda-control readiness. A measurement taken during another fuel mode may therefore answer a different question.

Learning conditions affect verification

The basic readiness criteria include coolant temperature, injection count and the absence of wide-open-throttle and catalyst-heating conditions. The summary contains more eligibility logic than those examples, so they are not presented as a complete universal drive cycle.

Retain bank 1's PFI correction and adaptation status together with fuel mode. The monitor establishes a learned lean-direction correction beyond its limit; it does not isolate the responsible air, fuel or measurement component. A valid recovery result needs the relevant adaptation to run again. Erasing learned values or observing a different fueling mode cannot independently establish that the cause has been repaired.

Source and scope

2025 GM 25OBDG07A Part 2 monitor tables only; model, engine and transmission applicability are not established by the unresolved application-matrix filename join. These are documented monitor criteria, not a universal repair procedure. GM PDF, physical page 223 · GM PDF, physical page 224.