P21DC: 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. 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.

Medium severity. Can I keep driving?Drive as little as possible and go to a repair shop. If you smell fuel or the engine is shaking badly, do not keep driving.

Cylinder 1 is numbered according to the manufacturer: check your engine's manual before working on it.

Symptoms

Check engine light on
Cylinder 1 runs weak or misfires
Torque limited if more cylinders fail (Mitsubishi)

Most likely causes

Injector control wire shorted to a supply wire
Both injector wires shorted together in the harness
Injector coil shorted internally
Driver stage of the control module damaged

What to check, in order

Start with the simplest. You can tick off each step.

Check the connector and wiring of the injector indicated
Check the sensor's ground and make sure the signal wire is not receiving voltage from another circuit.
Measure the resistance and compare it with the manufacturer's specification
Check the control signal with an oscilloscope; on direct injection or diesel engines, leave this to a repair shop because of the high voltages
On diesels, check the per-cylinder injection corrections

Good to know

Repair difficulty: hard
It can trigger limp mode.
It affects emissions.

What each character of P21DC means

P: Powertrain: engine and transmission
2: Generic code: same on every make
1: Air and fuel
DC: Specific fault in that system

Frequently asked questions

Can I keep driving with code P21DC?

Drive as little as possible and go to a repair shop. If you smell fuel or the engine is shaking badly, do not keep driving.

What causes code P21DC?

The most common causes are: injector control wire shorted to a supply wire; both injector wires shorted together in the harness; injector coil shorted internally.

Related codes

Sources

Values and procedures vary by manufacturer. If it affects brakes, steering, fuel or high voltage, see a professional. Updated: 2026-11-04. How we build these pages.