P21CAReductant Control Module Supply Voltage Circuit
- Generic OBD-II code · Powertrain
- Turns on the check-engine light
- Matters for the ITP
- Can trigger limp mode
- How the car detects it
- P21CA is the general code for the supply feed of the DEF control module: the voltage arriving at the module is missing or unusable, without the module assigning it to the low or high case. Volvo and Mack trucks use it, as P21CA00, on the aftertreatment control module that runs the SCR system. Volvo article KC-2040 describes a typical pattern: a battery or ground connection slowly loosens, the module voltage sinks below about 8 V, the module cuts power to the NOx, particulate and DEF sensors, communication codes such as U029D and U02A2 follow, and the truck enters SCR inducement and derate. Volvo stresses that this rarely means a bad module and first checks terminals, batteries, charging and the in-line fuse. P21CB and P21CC are the specific low and high codes.
- What you may notice
- Check engine light on
- Several sensor communication codes at the same time
- SCR inducement with power reduction
- Likely causes
- LikelyLoose or corroded battery or ground connections of the module
- PossibleWeak batteries or insufficient charging voltage
- PossibleFaulty in-line fuse or damaged supply wire
- RareControl module itself 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 – €600
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.