U0666 code meaning
Lost Communication With Reductant Pressure Sensor
Network system Generic code U06xx codes Check engine light
What U0666 means
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.
U0666 at a glance
- Check engine lightYes, usually on
- Emissions testCan fail
- Limp modePossible
- Repair difficultyMedium
- Typical labor0.5 to 4 hours
- Clears after repairWhen cleared with a scanner
Symptoms of U0666
- DEF system warning in the cluster
- Dosing stopped, higher NOx output
- Check engine light possible
Common causes of U0666
- Most 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)
- Less likelyFault in the dosing or engine control unit input
Check the most likely causes first. Replacing parts by guesswork is the most expensive way to clear a code.
Parts involved
Reductant pressure sensorVehicle bus wiring
What each part of U0666 means
| U | Network system: communication between the control modules (CAN bus). |
| 0 | Generic code: it means the same on every make (SAE standard). |
| 66 | The exact fault in that area. |
What to do next
- Note U0666 and any other codes stored with it: several codes together often point to one cause.
- Check for loose connectors and damaged wiring around the parts the code names.
- Check open recalls for your car: some faults are fixed for free by the dealer. Enter your VIN.
- Clear the code only after the repair. If it comes back, the fault is still there.
Buying a used car? Scan it for stored codes before you pay. A recently cleared memory can hide a problem.
Questions about U0666
What does U0666 mean?
U0666 means "Lost Communication With Reductant Pressure Sensor". The reductant pressure sensor watches the DEF line between the supply pump and the dosing valve. It is a generic code, so it means the same on every make.
Is it safe to drive with U0666?
Drive gently and only to a workshop. With U0666 the car can go into limp mode (reduced power) to protect itself.
Will U0666 fail an emissions test?
Yes, it can. U0666 is emissions-related, so most inspection programs fail a car while it is stored or while the check engine light is on.
How hard is U0666 to fix?
Difficulty: medium. A typical repair takes 0.5 to 4 hours of work. This is a job for a workshop.
Will U0666 clear itself after the repair?
Clear the code with a scanner after the repair. If the fault is fixed, it does not come back.