P21DC

Code

Generic P — Powertrain

Cylinder 1 Injector B Circuit High

Brand: Generic

Type: P — Powertrain

Views: UK: 19 EN: 29 RU: 28

AI status

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Page language: EN

Causes

Short to battery (constant 12V) on the injector control/driver circuit
Corroded, bent or damaged injector connector or pins
Internal short or failure inside the fuel injector
Poor or missing ground reference where applicable
Faulty ECM/injector driver transistor output
Aftermarket electrical modifications or shorts (alarms, stereo, etc.)

Symptoms

Check Engine Light / MIL illuminated
Rough idle or misfire on cylinder 1
Poor acceleration or reduced engine performance
Lower fuel economy
Possible hard start or stall in severe cases

What to check

Scan for P21DC and any additional related DTCs; record freeze-frame/live data
Visually inspect injector B on cylinder 1 connector and wiring for damage, corrosion, or loose pins
Backprobe the injector connector while key ON (engine OFF) to verify supply and control voltages
Measure injector coil resistance and compare to specification
Wiggle test wiring with connector backprobed to check for intermittent faults
If available, view injector waveform with a scope to confirm switching behavior

Signal parameters

Battery/supply voltage to injector: ≈ 11–14.5 V (with key ON/engine running) — verify exact spec
Injector coil resistance: vehicle-specific (common ranges: low-impedance ≈ 2–4 Ω; high-impedance ≈ 12–16 Ω) — check factory spec
Control signal: pulsed drive from ECM (pulse width varies with load, typically 1–10 ms) — duty varies 0–100%
When driver is active: control side should be pulled to ground or switched to battery depending on system; an abnormally high steady voltage indicates a short to power

Diagnostic algorithm

Retrieve and record freeze frame and all stored DTCs. Clear codes and perform key-on/engine-run to attempt to re-set and confirm repeatability.
Perform visual inspection of the cylinder 1 injector B harness and connector for corrosion, bent pins, melted insulation, or poor connections.
Backprobe the connector (supply and control terminals) with a digital multimeter: verify constant supply voltage on the power pin and monitor control pin for switching to ground (or appropriate behavior) when cranking/running.
Measure injector resistance at the connector (with harness unplugged) and compare to factory spec. Replace if out of range or open/shorted.
If resistance and bench checks are OK, use an oscilloscope to capture injector waveform while cranking/running. Look for abnormal voltage level (stuck high), missing switching, excessive voltage spikes, or distortion.
Perform a wiggle test of the wiring and connectors while monitoring the control signal or the DTC status to find intermittent faults.
Swap the suspect injector with the same type injector from another cylinder and clear codes. If the code moves to the other cylinder position, the injector is likely faulty. If it stays on cylinder 1, suspect wiring, connector, or ECM.
Check for shorts to battery: isolate the control wire and check continuity to battery voltage (with ignition off) to see if it is shorted. Repair damaged wiring or insulation.
If wiring and injector test good, test or replace the ECM driver for that injector per factory procedures (this often requires special tools or dealer support).
After repair, clear DTCs and road-test to confirm repair. Reinspect if code returns.

Likely causes

Wiring short to battery on cylinder 1 injector B control wire
Damaged or corroded injector connector at cylinder 1
Defective fuel injector with internal electrical fault
Faulty ECM output driver for that injector (less common)

Fault status

⚠️ Status

ECM detected high voltage on Cylinder 1 Injector B circuit. Diagnostic trouble code P21DC stored. Investigate injector wiring, connector, injector unit, and ECM driver.

🟡 Repair difficulty: Medium

⏱️ Diagnostic time: 1.0-2.5 hours