Diagnostic article Generic OBD-II
P21A0: cylinder-five imbalance can be hidden by a normal-looking bank average
GM P21A0 evaluates cylinder five relative to its bank, so a bank average and an individual-cylinder result answer different questions.
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Documented application
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.
The code alone does not identify a failed part or establish that a vehicle is safe to drive. Confirm the vehicle, reporting module and applicable service procedure.
P21A0 is a cylinder-five air-fuel imbalance code. In the cited GM strategy, the individual cylinder is judged against the bank's average air-fuel behavior. That comparison explains why a generally reasonable bank value does not rule out a meaningful difference in one cylinder.
Preserve the individual evidence
The table includes P21A0 in both lean-side and rich-side evaluations. The normalized comparison is not the same quantity as a scan tool's familiar short-term fuel-trim percentage. Record the available cylinder-specific result and the operating point instead of treating a single bank correction as proof of the cause.
The source does not identify an injector leak, a restricted injector or an intake leak as the inevitable explanation. Those possibilities require application-specific evidence. The fifth cylinder's physical location also depends on the actual engine, which this document-level assignment does not establish.
Distinguish an unavailable test from a passing test
The comparison relies on valid engine-roughness and segment-time information. It requires trigger-wheel adaptation, normal synchronization, no active rough-road detection and a calibrated speed/air-charge operating region. A monitor that was prevented from running cannot demonstrate that the original cylinder-five condition disappeared.
GM lists the evaluation once per driving cycle. Repair verification should therefore record whether an eligible evaluation actually occurred and whether associated faults remain. The source supports this interpretation of the monitor; it does not provide a universal component replacement order or a road-test recipe for every vehicle carrying P21A0.
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.
Article source documents
Source checks describe how the text was prepared; they do not certify a diagnosis or confirm compatibility with every vehicle using this code.
Original code reference
Dictionary definition
Cylinder 5 Air-Fuel Ratio Imbalance
This definition retains its original catalog scope. The article above is limited to its explicitly documented application.
Reference scope
Generic OBD-II definition.
A manufacturer label does not establish the model, year, engine or module to which a diagnosis applies. Read the safety and diagnostic limits.
Definition source
- DTC Database: pinned community code definitions
Source checked · MIT