P21CBReductant Control Module Supply Voltage Low
- Generic OBD-II code · Powertrain
- Turns on the check-engine light
- Matters for the ITP
- Can trigger limp mode
- How the car detects it
- P21CB says the DEF control module receives too little supply voltage. GM measures it as a difference to system voltage: in the 2018 diesel tables a gap above 3 V between battery voltage and the supply of the SCR power module, seen in 40 of 50 samples taken every 100 ms with the powertrain relay in range and cranking excluded, sets the code; the 2025 tables apply the same 3 V gap to the permanent DEF supply against the gateway voltage for 3 s and classify it as Type B. Ford's 2023 diesel summary uses an absolute limit for its reductant pump controller, below 8.5 V within 5 s. Causes lie between battery and module: resistance in fuses, terminals or ground. P21CA is the umbrella code, P21CC the overvoltage case.
- What you may notice
- Check engine light on
- DEF dosing or heating drops out intermittently
- Further undervoltage or communication codes from the SCR system
- Likely causes
- LikelyVoltage drop at fuse, terminal or ground of the module feed
- PossibleWeak battery or low charging voltage
- PossibleDamaged or corroded supply wire to the module
- RareInternal voltage measurement of the module faulty
- Inspection (ITP)
- Matters for the ITP. While the check-engine light is on, the car fails the emissions check.
- Repair
- Medium · Often possible to do yourself.Typical cost: €0 – €500
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.