No 4 EGT Sensor Question

[mrcheeezy]

2014 2.0 A5 TDI with CJCB Engine manual gearbox Vin WAUZZZ8T9EA055755

So my son's A5 has just had it clutch changed by a clutch specialist. About 2 days and 50miles later the DPF warning came on (has never come on in 5 years of owning the car and never had any region issues). I checked VAG fault codes and No 4 EGT Sensor was showing implausible signal and was reading 437deg C when engine was cold (16 deg c ambient). Sensor 1 and 2 both read correctly within a degree of each other. Drove the car to try and get it to regen, DPF temp rose to 460 deg c and soot value started to reduce but the temp kept falling back to about 430 deg c and the regen stopped. When the car was warm no 1 and 2 EGT sensors both read about 550 to 600 deg C but no 4 then read 280 deg C? Why would it have gone down when warm? Tried a service regen which I ran for 40 mins and then stopped but this had no effect on soot values.

I replaced no 4 EGT sensor, plugged it in but got exactly the same readings as the old one. Checked the resistance of 4 EGT sensor and it reads 215 oms at ambient which I think is correct. So I'm thinking the cable to no 4 EGT (attached to the gearbox) has been damaged in the clutch change? Is there any way I can confirm this - why would the temp reduce as the exhaust warmed up? How can I check the cable as inaccessible to view (any multimeter reading I could check at the terminal?) Can I bypass the cable running another from where it joins the loom or ECU? Can someone confirm the part umber for no 4 EGT sensor as the one fitted looked like an aftermarket one and I ordered a replacement based on that number so it could be the wrong sensor?

Any help much appreciated - this is looking tricky????

[Crasher]

03l 906 088 dr