What it means
The control module monitors the voltage supplied to the reductant control module. It sets this code when the measured supply voltage exceeds the calibrated upper limit.
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 uses its supply voltage to manage DEF delivery and emissions treatment. P21CC sets when the monitored module supply exceeds its calibrated upper limit, which can result from a charging fault or a problem in the module's supply or ground circuit.
Common causes
Most likely first.
- Charging system overvoltage
- Poor battery or ground connection
- Voltage regulator fault
- Supply circuit wiring or connector fault
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 scan for charging-system and reductant codes to see whether the high voltage coincided with other electrical faults.
- Check battery and charging-system operation for overvoltage, including whether the problem occurs only while the engine is running.
- Inspect the module's supply wiring, connectors, and grounds for damage, looseness, corrosion, or signs of overheating.
- Measure system voltage and the module supply under the conditions specified by the manufacturer, comparing results with the manufacturer's specification.
- If charging voltage is normal but module supply remains high, test the supply and ground wiring for circuit faults using the manufacturer's procedure.
Common fixes
- Repair a charging-system fault that is causing overvoltage.
- Clean and secure a poor battery or module ground connection.
- Repair damaged or loose supply wiring and connectors.
- Replace a faulty voltage regulator if testing confirms it is responsible.
Can I keep driving?
The vehicle may remain driveable, but excessive voltage can damage electrical components, including reductant-system parts. Limit driving and have the charging and module supply checked promptly; the DEF warning may lead to a no-start countdown if emissions treatment is disabled.
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.