U0666: Lost Communication With Reductant Pressure Sensor

What it means

The vehicle bus reports loss of communication with smart reductant (DEF/AdBlue) pressure sensor. Pump pressure feedback for the SCR dosing system is unavailable.

At a glance

• Usually lights the warning light of the system that lost communication
• Affects emissions; the car can fail an emissions test

How it works

The reductant pressure sensor reports pressure in the DEF/AdBlue supply used by the selective catalytic reduction system. U0666 sets when its network messages are lost, leaving the dosing system without normal pump-pressure feedback and potentially triggering an emissions warning or dosing restriction.

Common causes

Most likely first.

1. Bus wiring open or shorted
2. Module without power or ground
3. Module failed

Symptoms

• Multiple warning lights may illuminate
• Affected modules may go into limp mode or shut down
• Specific functions inoperative depending on module
• Diagnostic tools may show communication faults

How to diagnose it

1. Scan the engine and aftertreatment modules, save freeze-frame data, and check for related reductant pump, dosing, or network codes.
2. Review live data for reductant pressure and sensor communication status, and note whether dosing or an emissions countdown warning is active.
3. Inspect the reductant pressure sensor connector and harness for corrosion, moisture, crystallised DEF, physical damage, or loose terminals.
4. Check the sensor's specified power and ground and verify any shared supply identified by the wiring diagram.
5. Test the sensor network circuit for opens, shorts, and abnormal resistance following the manufacturer procedure.
6. If the supply and network wiring pass, follow the manufacturer's reductant pressure and sensor communication tests before replacing the sensor.

Common fixes

• Clean and secure the reductant pressure sensor connector if contaminated by moisture or DEF residue.
• Repair damaged or corroded sensor wiring and terminals.
• Restore the sensor's failed power or ground feed.
• Repair an open or short in the network wiring to the pressure sensor.
• Replace the reductant pressure sensor if its power, ground, and bus circuit test correctly.

Can I keep driving?

You can usually drive if the vehicle operates normally, but the SCR system may stop dosing correctly and emissions warnings or a later restart restriction may follow. Address the fault promptly, especially if a DEF or inducement countdown is displayed.