Diagnostic article Generic OBD-II
P21A3: cylinder-eight imbalance needs a cylinder result and a bank reference
GM P21A3 monitors cylinder eight against its bank and requires valid timing-related inputs before the comparison can be trusted.
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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.
P21A3 is the cylinder-eight air-fuel imbalance code in this family. The cited GM strategy compares a cylinder's calculated air-fuel behavior with its bank average. An overall bank value and the cylinder-specific comparison should therefore be retained together when reviewing the event.
Do not mistake a prerequisite for a failed component
The monitor needs valid engine-roughness information, plausible segment duration and completed trigger-wheel adaptation. Normal engine synchronization is also required. These statements describe when the calculation is usable; they do not mean that the presence of P21A3 diagnoses a timing-wheel or crankshaft-sensor failure.
Active rough-road detection and an operating point outside the calibrated speed/air-charge region can prevent evaluation. A subsequent drive or idle period without the code may simply lack a qualifying monitor opportunity.
Establish which side of the comparison failed
Both rich and lean branches include P21A3. The generic code label does not identify the direction, so a repair proposal should explain the actual cylinder-eight evidence before selecting tests or parts. The source does not prescribe an injector replacement or a universal compression, fuel-pressure or resistance limit.
Cylinder numbering and vehicle applicability must come from the exact engine's documentation. After a confirmed cause is corrected, verify that the shared monitor's prerequisites are satisfied and that an eligible evaluation has occurred. Because the table lists a once-per-driving-cycle comparison and a two-trip classification, fault-memory clearing alone is a particularly poor substitute for documented post-repair evidence.
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 8 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