What it means
Supply voltage at the dedicated DEF quality sensor module is below the lower threshold.
At a glance
- Usually lights the check-engine light
- Affects emissions; the car can fail an emissions test
How it works
The reductant quality module evaluates DEF quality so the emissions system can respond if the fluid is unsuitable. P21CD sets when the supply voltage to this dedicated quality module falls below its lower threshold, without identifying where the voltage is lost.
Common causes
Most likely first.
- Weak battery or low system voltage
- Voltage drop in module power wiring
- Poor battery or ground connection
- Blown or high resistance fuse
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 battery, charging, and other reductant codes to see whether a low-voltage event affected multiple systems.
- Inspect the DEF quality module connector and nearby wiring for corrosion, moisture, looseness, or physical damage.
- Check the relevant fuse and its connections for an open fuse or poor contact.
- Measure power and ground at the quality module under the conditions specified by the manufacturer and compare results with the manufacturer's specification.
- If voltage is low at the module, test the fuse-to-module power path and ground for voltage drop or an open circuit.
Common fixes
- Clean and secure the quality module's power or ground connection.
- Replace a failed fuse after checking for the cause of failure.
- Repair a damaged, open, or high-resistance supply or ground wire.
- Repair a corroded or loose connector terminal.
Can I keep driving?
The vehicle may continue to run, but the system may be unable to verify DEF quality and can display a warning or start a no-start countdown after the ignore period. Avoid unnecessary driving and resolve the voltage fault promptly.
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.