P2178: A rich off-idle diagnosis based on bank 1’s learned correction

P2178 reports system too rich off idle on bank 1 in the generic catalog. In the cited GM calibration, the failure is the multiplicative part of long-term fuel trim falling below 0.73 while the relevant gasoline-mode adaptation is active.

A learned factor is different from an instantaneous correction

The monitor specifies a 0.2-second evaluation period, but only after its stability and similar-condition logic is satisfied. The continuation considers changes in the long-term factor, short-term correction and the additive part held at the start of the steady phase.

Consequently, one low trim value during acceleration or another transition cannot be assumed to reproduce the diagnostic. The controller is evaluating a particular learned correction under qualified conditions, not merely observing any momentary rich indication.

Keep the direction and data representation clear

A factor below 0.73 represents substantial downward correction relative to a unity factor. Some tools present trims as percentages instead, so the displayed quantity must be identified before numerical comparisons are made. The source does not establish that every vehicle with P2178 uses this exact boundary.

Useful evidence includes bank 1's multiplicative adaptation, its stability state, load and the associated short-term and additive values. The code does not identify an injector leak, an airflow error or a specific fuel-system component without additional evidence. Verification should show that adaptation and the monitor completed successfully; resetting learned values alone cannot demonstrate that the original cause was removed.

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 179 · GM PDF, physical page 180.