Diagnostic article Generic OBD-II
P21A1: evaluate cylinder six during an eligible mixture-comparison window
P21A1 requires an eligible GM cylinder-imbalance evaluation; fault clearing or a short idle check does not establish a passing result.
Read before applying this article
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.
P21A1 concerns cylinder-six air-fuel imbalance. The GM table compares cylinder-specific behavior with a bank average and lists the code in both rich and lean branches. The number identifies the cylinder addressed by the code; additional data must establish the direction and actual cause.
Operating range is part of the diagnosis
The monitor is enabled only in calibrated combinations of engine speed and relative air charge. The document also requires normal synchronized operation and valid engine-roughness information. Active rough-road detection or invalid segment timing can prevent the relevant comparison.
These conditions explain why an idle-only inspection may be inconclusive. They do not justify forcing the vehicle through unsafe operating conditions. The correct service procedure should determine how a technician obtains a valid test opportunity for the exact engine and calibration.
Read the result at the right level
A bank-wide mixture correction is not the same as cylinder six deviating from that bank. Likewise, the table's normalized thresholds should not be presented as injector resistance, fuel pressure or fuel-trim limits. Preserve the actual cylinder evidence and the bank reference used by the controller.
The source lists a once-per-cycle evaluation with a two-trip classification, so the meaning of a fresh code-clear history must be considered. Clearing P21A1 neither repairs a mixture imbalance nor proves that the monitor has rerun. After the cause has been established and corrected through the engine-specific procedure, verification should include a completed eligible comparison and resolution of the original complaint.
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 6 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