P21F3 code meaning
Cylinder 12 Injector B Circuit High
Powertrain system Generic code P21xx codes Check engine light
What P21F3 means
P21F3 ends the B injector series of SAE J2012: a high signal on the cylinder 12 circuit, caused by a short to supply on the switched side. The series runs from P21CF to P21DA for opens and from P21DB to P21F3 for low/high pairs; the A injector equivalent here is P0295. Public manufacturer documents only reach cylinder 8 (GM's LT5 V8) or cylinder 4 (Mitsubishi), and no twelve-cylinder with dual injection could be documented, since Lamborghini's and Aston Martin's V12s use port injection alone. Should the code appear, the injector cannot open; cylinder 12 then relies on its other injector, and the driver stage takes the brunt of each switching attempt. GM's criteria for the same fault type serve as a guide: within 0.5 Ohm of power with the driver off. Low fault: P21F2.
P21F3 at a glance
- Check engine lightYes, usually on
- Emissions testCan fail
- Limp modeNot expected
- Repair difficultyHard
- Typical labor1 to 5 hours
- Clears after repairWhen cleared with a scanner
Symptoms of P21F3
- Check engine light on
- Cylinder 12 lean or misfiring (P0312)
Common causes of P21F3
- Most likelyControl wire of the cylinder 12 B injector shorted to supply
- PossibleInjector coil with shorted windings
- Less likelyDriver stage damaged by a lasting short
Check the most likely causes first. Replacing parts by guesswork is the most expensive way to clear a code.
Parts involved
Fuel injectorInjector wiringEngine control module
What each part of P21F3 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. |
| F3 | The exact fault in that area. |
What to do next
- Note P21F3 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 P21F3
What does P21F3 mean?
P21F3 means "Cylinder 12 Injector B Circuit High". P21F3 ends the B injector series of SAE J2012: a high signal on the cylinder 12 circuit, caused by a short to supply on the switched side. It is a generic code, so it means the same on every make.
Is it safe to drive with P21F3?
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 P21F3 fail an emissions test?
Yes, it can. P21F3 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 P21F3 to fix?
Difficulty: hard. A typical repair takes 1 to 5 hours of work. This is a job for a workshop.
Will P21F3 clear itself after the repair?
Clear the code with a scanner after the repair. If the fault is fixed, it does not come back.