U0666Lost Communication With Reductant Pressure Sensor
- Generic OBD-II code · Network
- Turns on the check-engine light
- Matters for the ITP
- Can trigger limp mode
- How the car detects it
- The reductant pressure sensor watches the DEF line between the supply pump and the dosing valve. Bosch, for instance, offers its supply module with or without such a sensor; with it, the pump only runs when dosing is requested, the line stays pressurised through stop-start phases and is purged at key-off against freezing. If the sensor speaks a digital protocol, U0666 means its messages have stopped reaching the controller. No manufacturer documents this code in public. CARB places each SCR sensor under its comprehensive component rules and demands both a dedicated code for a silent digital input and detection of faulty reductant delivery. Analog faults are P204A to P204E, pressure too low or high P20E8 or P20E9. Dosing usually stops, followed by DEF warnings and possibly a MIL.
- What you may notice
- DEF system warning in the cluster
- Dosing stopped, higher NOx output
- Check engine light possible
- Likely causes
- LikelyDEF crystals or corrosion in the supply module connector
- PossibleHarness to the tank or pump module damaged
- PossiblePressure sensor failed (often only with the pump module)
- RareFault in the dosing or engine control unit input
- Inspection (ITP)
- Matters for the ITP. While the check-engine light is on, the car fails the emissions check.
- Repair
- Medium · Usually a job for a workshop.Typical cost: €80 – €1,500
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.