P0425 - limp mode

[dpround]

I have P0425, engine dropping in to serious limp mode - 30mph max and sometimes the ECU blocks cranking of the engine if stopped. I think the oxygen sensor or wiring in front of the cat may be to blame, but in this euro 5 van with the close coupled cat I can barely touch the sensor so cannot do any useful inspection of the wiring etc. Am I missing something? The access is almost too horrible to be true. It might be possible remove the cat from the engine as held on with a magic clip? Any advice appreciated.

[GiveMeABreak]

You seem to be missing the last 2 parts of the fault code. Without these I can't provide the exact details. There should be a 16, 17 or 27 on the end. All these faults relate to the particle filter upstream temperature sensor and not the Catalytic converter.

P0425 16 Exhaust high temperature sensor: Voltage below the minimum threshold
P0425 17 Exhaust high temperature sensor: Voltage above a threshold
P0425 27 Exhaust high temperature sensor: Voltage variation outside the tolerance

Pop your VIN up and the last part of the P0425 code to ensure I have the exact engine management system (please post your VIN in full without spaces - it will be automatically masked from public view after submitting your post & will only be visible to staff).

[dpround]

Excellent news. At least I can see the sensors in the dpf. Unfortunately the code has been cleared to get home. I'll try and recreate and read on a real reader this time.

[dpround]

I am unable to get a consistent resistance reading on the pre-DPF temperature sensor. The value I get is apparently cycling between 6 and 10M ohm with the sensor at ambient and disconnected. (The cycling bit is probably irrelevant consequence of using a DVM and low sample rate, but is oddly consistent.) The replacement pattern part shows a consistent 6M ohm. I am going to be lazy and just replace and see if the fault goes away.

[dpround]

Well finally found time to drive the van and it is not fixed. (It is a motorhome, so not in regular use...) The fault codes are 2428-98 and 0425-17. Probably more telling is that the pre-DPF temperature remains at 60C all the time in the live data where the pre-cat temperatures rises rapidly and believably to a few hundred C. I'm pretty confident that the fault is with the pre-DPC temperature sensor, but it may be in the wiring or ECU so is going to be a bit tougher to fix. Any hints on how you would proceed would be very welcome.

[GiveMeABreak]

I did ask for the VIN previously, so I can ensure the codes are correct for the engine management system you have, then I can deal with the other fault code.

[dpround]

Sorry Marc, my memory for detail could be better. VIN is VF3**************[VIN obfuscated, can be read by forum staff]

All the best

David

[dpround]

The plot thickens further. The diagnostic software (MaxiECU) Gives two exhaust temperatures, pre CAT and pre DPF. From unplugging the sensors and replacing with a short or open circuit to give max and min readings it is clear that the pre-CAT temperature is from the pre-DPF sensor and the pre-DPF temperature is from the sensor in the middle of the DPF - also loom and ECU are good, which is a relief! This means the sensor that is faulty is not the one I have replaced! I have ordered a post-DPF sensor, but it has arrived with the pre-DPF plug, so no resolution yet.

I could create a replacement post-DPF sensor from the old pre-DPF sensor and the plug from the faulty post-DPF sensor (the post-DPF sensor reads open circuit, so might really be faulty) if anyone happens to know if the sensors are based around the same thermistor.

[dpround]

After replacing the correct sensor, all appears well. I am hopeful that it is actually fixed this time. I could have saved a lot of time by paying more attention to the live data and using using deliberate anomaly conditions to prove I really understood what I thought I was seeing rather than relying on the fault codes. Thanks for the help