This code indicates that the engine or emissions control module has lost its data communication with the reductant pressure sensor, which is part of the diesel exhaust fluid (DEF/AdBlue) system. The sensor monitors DEF pressure to ensure proper dosing into the exhaust stream. Without valid communication, the SCR system cannot confirm reductant delivery and may disable dosing.
The DEF system wiring runs under the vehicle near hot exhaust and road debris, making chafed, broken, or corroded wires the most common cause of lost communication with the reductant pressure sensor.
The sensor connector is exposed to moisture, salt, and vibration, which can cause pin corrosion, bent pins, or poor terminal contact that interrupts the communication signal.
The sensor itself can fail internally, losing its ability to transmit data over the communication bus. This is common on high-mileage diesel vehicles with original DEF system components.
On some vehicles the pressure sensor is integrated into or closely tied to the reductant pump assembly. Internal pump circuit failures can disrupt sensor communication.
The module that communicates with the sensor could have an internal driver or bus failure, but this is rare and should only be considered after all wiring, connector, and sensor checks are exhausted.
You can typically drive short distances, but the SCR system will likely be disabled and the vehicle may enter a derate or speed limitation mode after a set number of ignition cycles. Continuing to drive risks failing an emissions test and potentially triggering additional DPF or NOx-related codes.
Almost never. This code is overwhelmingly caused by wiring damage, connector corrosion, or a failed reductant pressure sensor. Replacing the engine computer or dosing control module without thorough diagnosis of the circuit is a common and expensive misdiagnosis.
Not directly. Contaminated or expired DEF can cause other SCR codes related to quality or dosing, but U0666 specifically refers to an electrical communication loss with the pressure sensor, which is a wiring or sensor problem rather than a fluid quality issue.
Moderately urgent. The vehicle will not immediately stop running, but the SCR system is disabled, the vehicle may derate, and you may not pass an emissions inspection. Prolonged driving with the system inactive can lead to additional exhaust or DPF codes.