P21A4: Cylinder 9 Air-Fuel Ratio Imbalance

What does P21A4 mean?

P21A4 is reserved for an imbalance at cylinder 9 and can therefore only appear on engines with ten or more cylinders, a small group of V10 and V12 petrol engines. No manufacturer documents the code publicly and no bulletin in the NHTSA index mentions it. CARB's rule (13 CCR 1968.2 (e)(6.2.1)(C)) still applies to these engines, and it allows a maker to store a bank code or even a misfire code instead when its hardware cannot single out a cylinder. With five or six cylinders feeding one front sensor, separating them needs extra effort, for example a frequency analysis tied to the engine cycle as in the Cummins patent, or an active test that changes the fuel of one cylinder at a time as VW does on its five-cylinder. Diagnosis follows the usual order: injector, intake sealing, compression of cylinder 9.

The code tells you which monitor failed, not which part is broken. Test before replacing parts.

Symptoms

• Check engine light on
• Possible P0309 with a large deviation

Common causes

• High Cylinder 9 injector clogged or leaking
• Medium Intake leak at cylinder 9
• Low Compression loss at cylinder 9

Likelihood is a general estimate across vehicles; your make and model may differ.