Fix for U029D "fault in communication with the denox system"

[muggeotto]

Hi,
I had this fault code and was able to fix it. If you have this fault code maybe you have the same fault. I take no responcibility for your actions, this is purely for info. C4 grand picasso 2016 1,6 bluehdi. The denox sensor is located aft of the particle filter. It is a sensor in the exhaust and it has an electronic module as part of the system. It is located approx 30 cm from the sensor. You need to remove the sensor and control module. It turned out that the control module had been taking in water. The location on the vehicle is not the best.
It is possible to take off the plastic cover with a screwdriver or chisel if you are careful. You will then see if there is water or erosion inside. The circuit board can be removed by carefully bending it up. The connectors for the wiring will stay in the housing. Mine was in a terrible state and one of the wire tracks on the board was eroded away. This caused the module not to respond. Resoldering with a wire was performed. Cleaning of all parts was done, specially the sliding connectors for the wires. Putting on som clearcote paint in the housing before reassembly. The plastic cover needs to be glued back on for sealing so it will last, but before you do that put everything back on the vehicle and try if the fix was a success or not. I got a denox heater fault after this and it was because of the pins connecting the board to the housing wires was not in good contact. So I had to open it up again to fix that.

[GiveMeABreak]

Hi and welcome.

Thanks for your post, it was very interesting to see and could explain a lot of issues!

[Paul-R]

Great first post. Thank you.

[Trickiedickie]

Very useful info.
Thanks!