P21DC code meaning
Cylinder 1 Injector B Circuit High
Powertrain system Generic code P21xx codes Check engine light
What P21DC means
P21DC is the high-side fault of the cylinder 1 B injector: the line carries voltage where the control unit expects it to be low, typically from a short to supply or between the two injector wires. On GM's 2019 LT5 V8 the FICM injection module looks at the low side of the supplemental injector while its driver is off; less than 0.5 Ohm toward power in 20 of 25 samples sets a Type A fault, so that injector can no longer be switched. Mitsubishi's 2022 Eclipse Cross, where B is the direct high-pressure injector, needs a short circuit lasting 2 s and begins diagnosis on the negative DI line. Its manual warns against measuring the injector terminals, because the direct injection driver works with high voltage. Fail-safe, symptoms and codes P21DB (low) and P21CF (open) are closely linked.
P21DC at a glance
- Check engine lightYes, usually on
- Emissions testCan fail
- Limp modePossible
- Repair difficultyHard
- Typical labor1 to 4 hours
- Clears after repairWhen cleared with a scanner
Symptoms of P21DC
- Check engine light on
- Cylinder 1 runs weak or misfires
- Torque limited if more cylinders fail (Mitsubishi)
Common causes of P21DC
- Most likelyInjector control wire shorted to a supply wire
- PossibleBoth injector wires shorted together in the harness
- PossibleInjector coil shorted internally
- Less likelyDriver stage of the control module damaged
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 P21DC 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. |
| DC | The exact fault in that area. |
What to do next
- Note P21DC 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 P21DC
What does P21DC mean?
P21DC means "Cylinder 1 Injector B Circuit High". P21DC is the high-side fault of the cylinder 1 B injector: the line carries voltage where the control unit expects it to be low, typically from a short to supply or between the two injector wires. It is a generic code, so it means the same on every make.
Is it safe to drive with P21DC?
Drive gently and only to a workshop. With P21DC the car can go into limp mode (reduced power) to protect itself.
Will P21DC fail an emissions test?
Yes, it can. P21DC 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 P21DC to fix?
Difficulty: hard. A typical repair takes 1 to 4 hours of work. This is a job for a workshop.
Will P21DC clear itself after the repair?
Clear the code with a scanner after the repair. If the fault is fixed, it does not come back.