Recurring o2 sensor fault

[Bodhi-z]

Hi gents;

I have a 2000 Z3 with the M43B19 engine. Head gasket went recently, decided to do it myself due to silly price quoted by local garage.
All went ok seemed straightforward, car now runs fine but engine management warning light came on.
I read the code using my basic scanner - P0135 pre-cat o2 sensor.
I fitted a new sensor and cleared the code, but it came straight back after a short test drive.
I found a comment somewhere on line that said sometimes this fault code can actually be related to the engine coolant sensor?
My scan tool is pretty basic but I plugged it in to have a look at live data, and the temperature being recorded by the engine coolant sensor was minus 32 degrees...pretty sure that wasn't accurate! The temperature being recorded by the radiator coolant sensor appeared to be correct, it started off at 30 degrees and rose to 60ish whilst I was watching.
Just wondered if anyone else has come across this - did my coolant sensor die or have I not plugged something back in correctly whilst doing the head gasket job?
I do recall when I removed the lower part of the air intake manifold that there were two plugs hiding under there that were hidden from view and it's possible I may have stretched the wires I suppose when I lifted up the manifold to remove it without seeing them. I know one of these plugs is for the engine coolant sensor, and there was another round shaped plug fairly close to it........is that round one anything which may be a factor?
Any comment/advice would be much appreciated, thanks.

 

[colb]

If that sensor is duff judging by what you are seeing it reporting then put a replacement in and see what the scanner sees then. Not at all sure it is connected to the exhaust sensor code you are getting. Pre Cat code you are seeing is for the heater circuit in the sensor, check the wiring as its a new one you fitted and should be ok. Pre Cat sensor codes are usually set by the car having an air leak somewhere in the intake or elesewher across the engines vaccum system from splits or holes in the rubber hoses and/or intake hoses from Maf to throttle body. Back to the engine temp, is your temp guage rock steady at the 12 o'clock position when the engine is up to temp? If its slow to warm up suspect the thermostat having failed in the open position, replacement if it has and whilst your in there put in a new water pump.

 

[Bodhi-z]

ok thanks for comment. I think I will order a replacement coolant sensor, and strip upper and lower parts of the intake manifold completely off again. I can then replace the coolant sensor, check the wiring and look for potential vacuum leaks when I reassemble.
I thought the head gasket replacement went too easy, something had to go wrong!!

 

[Bodhi-z]

Fixed it!!
Wiring fault - I went back under the car to have a look at the wiring. On the plug that the new o2 sensor plugs into - I had a tug on the wires that came out of the back of that plug. The brown one was broken, a 300mm length of the wire just pulled straight out of the outer sheath with a frayed end. So I went topside and cut open the outer sheath at the point where the sensor wires meet the engine bay wiring loom - fished out the broken wire and soldered back together - sorted and no fault codes now.
Nice easy fix in the end, phew............