W205 NOX/AdBlue/comms

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New Member
Sep 11, 2021

Car is 2014 C250 Bluetec AMG line.

A couple of years ago it went to the main dealer for a service. At collection time they told me they had to keep it overnight for a software update, very slow data rate I assumed. This was the diesel update.

A few weeks later the EML came on. Back to the dealer and both NOX sensors were replaced under warranty.

A few months later the EML was back on again. I took the car back to the dealer who investigated, found the driver floor pan had some water in it “due to non-OEM windscreen replacement”, diagnosed corroded contacts in the sensors and refused to do replace them. I asked if they would clean the contacts or replace the connectors they refused, the only option was to replace both at my cost. Performance and mpg were fine, I bought a code reader that diagnosed low voltage. I lived with the EML being lit, snubbed it for the MOT which was a pass so no emissions problem, and the car drove fine with consumption in the high 50-mpg range.

MOT due in two weeks so I thought I’d better take it a bit more seriously. I topped up the AdBlue, which hasn’t been touched for about two years with lockdown mileage, and set off on a long trip. About 150 miles in up came the “Ad Blue no start after 497 miles” warning. I cleared this and the residual NOX codes with the scanner. Filled the AdBlue tank and that message hasn’t shown again. However, P13DF is now showing when the EML lights one up to temperature. Also, P2200, P229E and P229F reported, which are voltages/ comms errors.

This weekend I removed the short lengths of wire harness between the NOX sensors and the male connectors where the cable enters the body. There was a minute amount of green corrosion product which I cleaned with a fibre glass brush and contact cleaning spray. Checked impedance with a multimeter, all good so these contacts seem ok. Refitted, ran the engine and eventually the EML lit again and codes UO29D, U029E, P13DF and P229F are indicated again. These seem to suggest AdBlue sensor fault and NOX comms again. I’m aware that the P13DF may be linked to/caused by NOX faults.

Like so many I’m at a loss to know what to do now. I’m loathe to take the car to the dealer because it seems inevitable I’ll be charged for two more new NOX sensors and AdBlue tank only to have the light come on a few weeks down the line because of a residual comm/cable fault.

I haven’t investigated any connections on the interior side of the floor pan, and am unsure if there are any. Can anyone advise? It might make sense to clean and check those contacts given the moisture problem.

It seems a bit bizarre that the AdBlue fault only cropped up once the tank was refilled, as if that part of the system re-awakened.

The optimist (diminishing) within me wonders if it needs to be plugged into STAR now the contacts have been re-made and everything recalibrated or whatever black magic it works.

I’d be grateful for any advice.


It is such a shame that the adblue system causes so many expensive problems as it spoils the ownership of an otherwise very good car.
From the dealer diagnosis I guess that there must be some contacts in the driver footwell - otherwise the dealer response does not make any sense?
I think that I would try a good MB independent such as PCS at Horndean and see if they can help. They are likely to take a more pragmatic approach rather than insisting on just replacing everything at your cost.

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