P21A2: cylinder-seven imbalance has rich and lean diagnostic branches

P21A2 identifies cylinder-seven air-fuel imbalance. It is easy to overread that label as a particular injector fault, but the cited GM table describes a bank-relative calculation. The diagnosis starts with the measured or modeled pattern, not with a predetermined replacement part.

The code does not supply mixture direction

P21A2 appears in both the too-lean and too-rich rows of the shared monitor. The lean comparison exceeds approximately 1.18, while the rich comparison falls below approximately 0.83. Those values are calibration-specific normalized quantities; they are not percentages to apply directly to a fuel-trim display.

The practical consequence is to retain the direction and the relevant bank reference before memory is cleared. A common bank condition and a deviation isolated to cylinder seven require different evidence, even if a simplified scanner displays the same broad mixture terminology.

Check the prerequisites before calling the repair successful

The calculation depends on completed trigger-wheel adaptation, plausible segment timing and synchronized normal engine operation. The monitor excludes active rough-road detection and requires an appropriate speed/air-charge region. Its absence during another operating state is not a passing result.

The document does not provide model fitment or the physical location of cylinder seven. Confirm both from the actual vehicle information. A workshop should investigate the evidenced pattern using that engine's procedure, then establish that the once-per-cycle comparison has completed appropriately. A cleared dashboard warning by itself cannot demonstrate that the cylinder-specific condition is gone.

Source and applicability

GM 2020 20OBDG07 shared cylinder air-fuel imbalance monitor. Only the cited monitor and calibration are described; per-code vehicle and engine applicability has not been established.

20OBDG07 Diagnostics, physical PDF pages 129, 130, 131, 132, 133, 134.