P21A6 Cylinder 11 Air-Fuel Ratio Imbalance

P21A6 assigns the imbalance to cylinder 11, which exists only on twelve-cylinder engines. No manufacturer has published how it detects this code, and no NHTSA bulletin names it, so it is a placeholder that J2012 provides for large engines. The legal frame is CARB's cylinder imbalance monitor (13 CCR 1968.2 (e)(6.2.1)(C)). A V12 or W12 feeds six cylinders into each front sensor, and exhaust pulses overlap, which makes a clean per-cylinder reading hard; the Cummins patent describes estimating the signal content at the engine cycle frequency and then trimming the fuel of the cylinder that causes it, an approach suited to many cylinders. In practice the work is mechanical: confirm the numbering, compare injector delivery across the bank and run a compression or leak-down test on cylinder 11.

Likely causes

Symptoms

• Check engine light on
• Possible P0311 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.