P21A5 code meaning
Cylinder 10 Air-Fuel Ratio Imbalance
Powertrain system Generic code P21xx codes Check engine light
What P21A5 means
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, gasoline 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.
P21A5 at a glance
- Check engine lightYes, usually on
- Emissions testCan fail
- Limp modeNot expected
- Repair difficultyHard
- Typical labor1 to 8 hours
- Clears after repairAfter a drive cycle
Symptoms of P21A5
- Check engine light on
- Possible P0310 or uneven idle
Common causes of P21A5
- Most likelyCylinder 10 injector out of tolerance
- PossibleFault in another cylinder of the same bank shifting the result
- Less likelyCompression loss at cylinder 10
Check the most likely causes first. Replacing parts by guesswork is the most expensive way to clear a code.
Parts involved
Fuel injector
What each part of P21A5 means
| P | Powertrain system: engine, fuel system, ignition, emissions and transmission. |
| 2 | Generic code (second SAE range): it means the same on every make. |
| 1 | Area: fuel and air metering. |
| A5 | The exact fault in that area. |
What to do next
- Note P21A5 and any other codes stored with it: several codes together often point to one cause.
- Check for loose connectors and damaged wiring around the parts the code names.
- Check open recalls for your car: some faults are fixed for free by the dealer. Enter your VIN.
- Clear the code only after the repair. If it comes back, the fault is still there.
Buying a used car? Scan it for stored codes before you pay. A recently cleared memory can hide a problem.
Questions about P21A5
What does P21A5 mean?
P21A5 means "Cylinder 10 Air-Fuel Ratio Imbalance". P21A5 flags cylinder 10, which on a V10 is the final cylinder and on a V12 sits near the end of its bank. It is a generic code, so it means the same on every make.
Is it safe to drive with P21A5?
Usually yes, for a short time, if the check engine light is steady and the car drives normally. If the light flashes or the car runs badly, stop and have it checked.
Will P21A5 fail an emissions test?
Yes, it can. P21A5 is emissions-related, so most inspection programs fail a car while it is stored or while the check engine light is on.
How hard is P21A5 to fix?
Difficulty: hard. A typical repair takes 1 to 8 hours of work. This is a job for a workshop.
Will P21A5 clear itself after the repair?
After the repair the car has to complete a drive cycle (a mix of city and highway driving) before the test runs again. Until then the readiness monitor shows "not ready", which can fail an inspection.