P21A3: Cylinder 8 Air-Fuel Ratio Imbalance

What does P21A3 mean?

P21A3 blames cylinder 8, on a V8 the last cylinder in the numbering. No manufacturer documentation of this code is publicly available, and no bulletin in the NHTSA index refers to it; the basis is the general CARB requirement for detecting cylinder imbalance, phased in for 2011 to 2014 model years with an emission threshold of 3.0 times the FTP standard (4.0 for SULEV II) until 2014 and 1.5 times from 2015. Whether cylinder 8 lies on bank 1 or bank 2 depends on the maker's numbering, which tells you whether P219A or P219B should accompany it. As Toyota and Lexus bulletins on cylinder codes show, leaking or restricted injectors, an intake leak at one runner and leaking valves are the usual causes. An injector balance test and a leak-down test on cylinder 8 come before any sensor work.

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

Symptoms

• Check engine light on
• Usually no driveability change
• Possible P0308 with a strong deviation

Common causes

• High Cylinder 8 injector out of tolerance
• Medium Valve leakage at cylinder 8
• Medium Intake leak at the cylinder 8 runner

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