What it means
The reductant control module power relay control circuit signal is high. The code alone does not confirm a short to B+; the cause depends on the circuit design.
At a glance
- Usually lights the check-engine light
- Affects emissions; the car can fail an emissions test
How it works
The reductant control module power relay switches power to the system that meters DEF into the exhaust aftertreatment system. The engine controller monitors the relay control circuit and sets P21C9 when its signal is higher than expected; the code does not by itself prove the wire is shorted to battery voltage.
Common causes
Most likely first.
- Short to B+
- Connector pins corroded - high contact resistance
- Supply wire pinched against B+ source
Symptoms
- Check engine light or NOx warning on
- AdBlue/DEF level or quality warning
- Possible no-start countdown after ignore period
- Failed emissions inspection (NOx)
How to diagnose it
- Record freeze-frame data and check related charging, battery, and reductant codes to see whether the high signal occurred with a broader electrical fault.
- With the ignition off, inspect the reductant relay control wiring and connectors for pinching against a power feed, damaged insulation, moisture, or corrosion.
- Check connector pins for corrosion or looseness, since poor contact can distort the signal even when the circuit is not shorted.
- Use a wiring diagram and scan-tool data to identify the control circuit, then measure its signal under the conditions specified by the manufacturer.
- If the signal remains high, isolate the relay and circuit sections using the manufacturer's diagnostic procedure to locate a wiring or component fault.
Common fixes
- Repair damaged or pinched relay control wiring.
- Clean or repair corroded or loose connector terminals.
- Repair a confirmed short to a power source in the relay control circuit.
- Replace a faulty reductant power relay if circuit tests identify it as the cause.
Can I keep driving?
The vehicle may remain driveable initially, but the DEF system may stop working and trigger a no-start countdown after the permitted ignore period. Avoid extended driving with the warning active and address it promptly to prevent emissions noncompliance or being unable to restart.
A code shows where the control module detected a fault, not always which part failed. Check the causes in order and confirm with a test before replacing parts. Have several codes? Check them together
This is a generic code: it means the same on every make. The car’s maker may add more detail in its own service information.