P21A4 Cylinder 9 Air-Fuel Ratio Imbalance

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.

Likely causes

Symptoms

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

Repair estimate

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

Related codes

References: law.cornell.edu, patents.google.com, static.nhtsa.gov.