OBD Cylinder 1 Injector B Circuit High
What it means
P21DC stands for Cylinder 1 Injector B Circuit High. P21DC is a P code, so the fault sits in the engine, transmission and emissions systems. For P21DC, the second digit places it in fuel and air metering. For P21DC, work through the cause list below, cheapest checks first.
Where it lives
For P21DC, the circuit described runs through the injector. The description of P21DC puts it on cylinder 1.
What to check first
For P21DC, check the wiring and connector on this circuit first, since a broken terminal gives the same reading as a failed part.
Check these first
- P21DC: Faulty or contaminated sensor in the fuel/air metering circuit
- P21DC: Damaged or shorted wiring or connector for the affected circuit
- P21DC: Vacuum leak or intake system restriction
- P21DC: Failing fuel pump, injector, or fuel pressure regulator
What to do next
P21DC is a fault in the fuel and air metering system. The steps below are general, so what actually failed on your vehicle may not be what P21DC names.
- P21DC: Read freeze frame data and related codes with an OBD-II scanner
- P21DC: Inspect the affected sensor wiring and connector for damage
- P21DC: Compare live sensor readings (fuel trim, airflow, temperatures) to manufacturer specs
- P21DC: Smoke test the intake system if lean or vacuum conditions are suspected
- P21DC: Replace the failed component after confirming it with live data
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Updated
Generic SAE J2012 definition. Last updated 2026-08-30.