P21A2 Cylinder 7 Air-Fuel Ratio Imbalance

P21A2 places a mixture imbalance at cylinder 7, so it only exists on engines with eight or more cylinders. No manufacturer documents this code publicly; the bulletin index of the US safety authority NHTSA contains no bulletin that names it. The requirement behind it is CARB's monitor for uneven mixture between cylinders (13 CCR 1968.2 (e)(6.2.1)(C)), which asks the code to identify the cause as far as the method allows. A V8 has four cylinders per front sensor, so the sensor signal alone separates them less clearly than on a four-cylinder; methods that work at the engine cycle frequency, as described in a Cummins patent, or VW's individual leaning test make cylinder-level codes possible. Check the cylinder 7 injector, its intake sealing and compression, and compare with misfire counts.

Likely causes

Symptoms

• Check engine light on
• Often no noticeable symptom
• Possible P0307 if the deviation is large

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.