OBD Reductant Control Module Power Relay/Relays Control Circuit High
What it means
P21C9 stands for Reductant Control Module Power Relay/Relays Control Circuit High. P21C9 is a P code, so the fault sits in the engine, transmission and emissions systems. For P21C9, the second digit places it in fuel and air metering. For P21C9, work through the cause list below, cheapest checks first.
Where it lives
For P21C9, the relay sits on the circuit this code reports. The description of P21C9 reports a fault at the relay.
What to check first
For P21C9, pull the connector and inspect the terminals for spread, corrosion or backed-out pins before testing anything else.
Check these first
- P21C9: Faulty or contaminated sensor in the fuel/air metering circuit
- P21C9: Damaged or shorted wiring or connector for the affected circuit
- P21C9: Vacuum leak or intake system restriction
- P21C9: Failing fuel pump, injector, or fuel pressure regulator
What to do next
P21C9 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 P21C9 names.
- P21C9: Read freeze frame data and related codes with an OBD-II scanner
- P21C9: Inspect the affected sensor wiring and connector for damage
- P21C9: Compare live sensor readings (fuel trim, airflow, temperatures) to manufacturer specs
- P21C9: Smoke test the intake system if lean or vacuum conditions are suspected
- P21C9: Replace the failed component after confirming it with live data
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Updated
Generic SAE J2012 definition. Last updated 2026-08-30.