What does code U029D mean?
Symptoms seen in the workshop
- MIL on
- AdBlue or emissions warning on the cluster
- No-start countdown ("start not possible in X miles") on many Euro 6 vehicles
- AdBlue dosing disabled, followed by P20EE
- Limp mode / reduced power
Common causes of code U029D
- NOx sensor removed or unplugged after deleting the SCR/AdBlue system
- Faulty NOx sensor (internal failure of the sensor control unit, very common)
- No power supply: NOx sensor fuse or relay
- Sensor CAN wiring damaged by heat or chafing near the exhaust
- Corroded or wet connector
- Incompatible replacement sensor (different part number or software version)
Fix by ECU remapping
ECU remapping solution when the SCR system has been removed or the sensor will not be replaced:
1. AdBlue/SCR OFF + NOx OFF (calibration): deactivation of AdBlue dosing, SCR monitoring and CAN supervision of the NOx sensors, including the no-start countdown.
2. DTC OFF: deactivation of U029D and associated codes: U029E, P2200, P2201, P229F, P20EE and P207F.
If the SCR system is still fitted and must be kept, diagnose power supply, wiring and sensor instead: disabling U029D on its own leaves the SCR unmonitored.
⚠️ Important: Disabling the DTC without the correct calibration of the related system (AdBlue OFF + NOx OFF + DTC OFF) is not a valid fix. Both the calibration maps and the diagnostic mask have to be modified.
