Powertrain, generic (extended)

P21A5 Cylinder 10 Air-Fuel Ratio Imbalance

Check engine lightOn
EmissionsRelevant
Drive cycleRequired
Limp modeNo

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.

Likely causes

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

Symptoms

  • Check engine light on
  • Possible P0310 or uneven idle

Repair estimate

DifficultyHardWorkshop job
Parts and labour€0–2,500
Time1–8 h

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

Parts involved

fuel injector

Related codes

P219AP219BP21A4P21A6P0310

References: law.cornell.edu, patents.google.com.