P21A5: Cylinder 10 Air-Fuel Ratio Imbalance

What does P21A5 mean?

P21A5 flags cylinder 10, which on a V10 is the final cylinder and on a V12 sits near the end of its bank. Public manufacturer material for this code does not exist, and the NHTSA bulletin index has no entry for it. CARB requires detection of any cylinder imbalance that lifts emissions above one and a half times the FTP limit (from 2015) and that the fault code point as precisely as the method allows; since a phase-in from 2019 to 2021, petrol vehicles must also log how often the check actually runs in daily use. Before chasing cylinder 10 specifically, confirm that the flagged number matches the manufacturer's numbering and that no other cylinder of the same bank also shows a misfire count or a deviating injector, because a correction on one cylinder can shift its neighbours. Then test the cylinder 10 injector and compression.

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

Symptoms

• Check engine light on
• Possible P0310 or uneven idle

Common causes

• High Cylinder 10 injector out of tolerance
• Medium Fault in another cylinder of the same bank shifting the result
• Low Compression loss at cylinder 10

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