OBD-II code · Powertrain (generic, extended)
P21CD Reductant Quality Module Supply Voltage Low
Supply voltage at the dedicated DEF quality sensor module is below the lower threshold.
Is it safe to drive with P21CD?
Usually safe to drive in the short term. Get it diagnosed when convenient, sooner if the car drives differently.
Common causes
- Most likelyVoltage drop in supply harness
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)
Repair at a glance
- Difficulty
- Medium
- DIY friendly
- Best left to a mechanic
- Typical labour
- 0.5–3 h
- Emissions test
- Likely to fail
- Parts involved
- Reductant Quality Sensor
Code data: OBDex open dataset (CC0). Labour times are typical ranges, not a quote.