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U0666 Lost Communication With Reductant Pressure Sensor

Check engine lightOn
EmissionsRelevant
Drive cycleNot required
Limp modePossible

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.

Likely causes

DEF crystals or corrosion in the supply module connector
Harness to the tank or pump module damaged
Pressure sensor failed (often only with the pump module)
Fault in the dosing or engine control unit input

Symptoms

  • DEF system warning in the cluster
  • Dosing stopped, higher NOx output
  • Check engine light possible

Repair estimate

DifficultyMediumWorkshop job
Parts and labour€80–1,500
Time0.5–4 h

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

reductant pressure sensorvehicle bus wiring

Related codes

P204AP204BP204CP204DP204EP20E8P20E9U0619U061C

References: law.cornell.edu, bosch-mobility.com.