P21F9: Understanding a rich idle adaptation on bank 1’s PFI fuel path

P21F9 is the bank 1 fuel-control system B too rich at idle code. In the GM monitor, this is the lower-limit diagnosis for additive adaptation on the port-fuel-injection path. The code reports a learned correction outcome rather than directly identifying a failed injector.

The adaptation sequence must be eligible

The boundary is an additive correction below −5.484 in the document's internal scale. The source requires additive-integrator stability after the PFI multiplicative integrator has stabilized. The continuation includes a six-second stable check for the multiplicative path and further conditions on its change.

Pure port injection must also be active for more than two seconds. Therefore, an otherwise normal observation while another injection mode is operating does not establish that the PFI adaptation recovered.

Keep the correction scale and cause separate

The internal value is not specified as a standard scanner percentage. Before comparing numbers, establish that the tool is displaying the same adaptation component for bank 1 and the same fuel path.

The readiness list includes coolant temperature, injection count and qualified lambda operation. Those prerequisites explain why a reset or brief operating sample is not a complete diagnostic pass. Preserve the correction, stability and mode information, then use the applicable system procedure to investigate why the adjustment was needed. The source supplies no basis to rank a leaking injector, pressure problem or sensor fault as the cause without further evidence.

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 228 · GM PDF, physical page 229.