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[JamesV6CDX]

It may well be a red herring. But if the CTS is buggered it throws a lot of unknowns into the mix - and as MDTM often says - must fix the known problems first

[Doctor Gollum]

It has a common connection with an active code.

I would put money on it being a trapped wire between the O2 sensor and the ECU, and would focus on the Grey wire.

Would have offered to investigate, but I am pretty tied up until the end of April 

[Doctor Gollum]

As an aside, Eurocraparts list no sensors for OU54AEW.

[Migalot]

I’ve an appointment on Thursday. Free tomorrow afternoon?  I can possibly make that work

Best not tomorrow as the car is not outside and I have to go and get some provisions.
Wednesday could be on as the teachers on strike both Wednesday and Thursday.

[JamesV6CDX]

Thursday should be okay. I'll confirm for sure by morning

[JamesV6CDX]

Out of interest, what Lambda sensor did you fit?

When the lambda fault code is cleared, does it return immediately, or does it take a while of driving before it comes back?

[Migalot]

Out of interest, what Lambda sensor did you fit?

When the lambda fault code is cleared, does it return immediately, or does it take a while of driving before it comes back?

NGK sensor (there were no Bosch available at all in the UK). It won't let me clear the O2 heater circuit and malfunction, but the others go and the light goes out.

[JamesV6CDX]

Out of interest, what Lambda sensor did you fit?

When the lambda fault code is cleared, does it return immediately, or does it take a while of driving before it comes back?

NGK sensor (there were no Bosch available at all in the UK). It won't let me clear the O2 heater circuit and malfunction, but the others go and the light goes out.

This does point slightly away from the CTS causing the lambda issue... because I wouldn't expect the code to return until the engine was restarted -  BUT there is still a chance the dodgey CTS could have caused a good lambda sensor to fail if it's kept the heater element constantly on for hours at a time.

In any event - that CTS is goosed if it's reading 42deg at ambient temperature, and even on it's own could cause rough running and poor fuelling. So it needs to be changed regardless  (especially for £13) which affords a good opportunity whilst there to inspect the wiring back to the ECU...

We will get there, we really will

[Doctor Gollum]

Surely it would run the sensor heaters in ALL the exhaust sensors... 🤔

[JamesV6CDX]

Surely it would run the sensor heaters in ALL the exhaust sensors... 🤔

What is the trigger event for the ECU to stop running the heater(s)? Is it, a certain coolant temp threshold, temperature in the exhaust, some form of feedback from lambdas individually? If it's not the latter, then I agree with you - if it is then it makes things all the more curious....

[Doctor Gollum]

The heaters are fed from the ecu individually and return through a common wire that runs through the CTS. (as well as the cam sensor and mass air sensor).

The cam sensor is on/off, so as long as it sees a signal, no issue.

The MAF is variable and cross references with the throttle position, and again no code so is apparently within limits.

The CTS doesn't trigger an error within the ecu, ie it is within reason. If the car had been driven upto temp within the preceding few hours, then the coolant could easily have been at 42°. No mention of this, but Migalot did say that it wasn't from overnight stone cold. And unless the temp is verified by a secondary check such as the gauge, and or hands on, there's a fair chance the ecu sensor is reading correctly. (from experience, when these fail, they tend to go full Siberia mode and read - 40°, which increases the injector times enough to flood a remotely warm engine in a UK winter).

Cold start enrichment will happen as a result of intake air temp/external temp and possibly coolant temp, and at idle will be active for several minutes unless the throttle is pressed. Don't forget, it's sole purpose is to get the cats upto temp.

The O2 sensors contain two circuits... Heater and sensor.

The sensor side is reporting normally, ie no fault.

The heater side is reporting a fault on one consistent sensor. Replacing this three times has not changed the fault at all.

Ergo, the fault is in the wiring.

[Kevin Wood]

The Lambda sensor heater will be monitored for resistance by the ECU, so it's probably a wiring fault, not related to the CTS, in my view.

Lambda sensor heaters are normally PTC heaters that stabilise their own temperature, so are only connected to the ECU for monitoring purposes.

First job for me would be to check the resistance of the heater lines between ECU and lambda sensor socket.

[JamesV6CDX]

The Lambda sensor heater will be monitored for resistance by the ECU, so it's probably a wiring fault, not related to the CTS, in my view.

Lambda sensor heaters are normally PTC heaters that stabilise their own temperature, so are only connected to the ECU for monitoring purposes.

First job for me would be to check the resistance of the heater lines between ECU and lambda sensor socket.

Yes, as soon as I read that the fault won’t clear with the engine still off, I had doubts about the CTS causing this issue (although it does still appear to be faulty based on we know).

What resistance would you expect to see, Kevin?

[JamesV6CDX]

The Lambda sensor heater will be monitored for resistance by the ECU, so it's probably a wiring fault, not related to the CTS, in my view.

Lambda sensor heaters are normally PTC heaters that stabilise their own temperature, so are only connected to the ECU for monitoring purposes.

First job for me would be to check the resistance of the heater lines between ECU and lambda sensor socket.

Yes, as soon as I read that the fault won’t clear with the engine still off, I had doubts about the CTS causing this issue (although it does still appear to be faulty based on we know).

What resistance would you expect to see, Kevin?

That was a stupid question by me, please ignore it. I was thinking about measuring ohms across an injector or similar, not testing for end to end continuity. Wake up James 

[Doctor Gollum]

Resistance down the wire from damage will be higher than a good wire. So comparing the resistance of all four wires between the plug and the ecu will confirm damage in a given wire. I suspect you will find the green wire has a slightly higher resistance to the others.

Continuity or lack off/open circuit will indicate a break but not internal damage... There being a subtle distinction