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Is my diagnosis right?

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diagnosis
1K views 8 replies 5 participants last post by  FoxtrotOscar 
#1 ·
Hi,

My 2001 y reg 156 1.8TS has been dispplaying the engine warning light so I got a fault code reader and checked it out. It's reporting 1 fault code only:
P0115 - Engine Coolent Temerature Sensor 1 Curcuit (not sure if the one goes in there)

The only sympton the car has overall is the temperature guage doesn't move over 50 (or sits at 50 which is the bottom). After a 30 mile drive on the A303 and then A roads the real time reported Engine Coolent Temerature was 58degrees C after about 10 minutes from stopping. I've not been able to get any warm air from the vents either while on the drive for over 30 mins.

My hunch is that it's the thermostat has failed, could anyone confirm this based on the above description. If so, is the thermostat replacement something I could attempt myself, is there a "how to" anywhere?

Thanks
Ed
 
#2 ·
#3 ·
Yep cleared the code and it didn't come on with a restart but it went off last night by itself also... which was odd. It came back on on this morning commute though so I'll see if it comes back tomorrow morning also as it's been cleared now.

Would I not get a fault code if the temerature is above or below a certain threshhold?
 
#5 ·
Nope you won't get a MCSF (unless the temp goes about 120 IIRC) sounds like you have a dodgy connector on the sender, unplug it clean it with some electrical contact cleaner and reseat ...

the sender is basically a couple of independent thermistors (one for the gauge and one for the ECU) they should be about 600 Ohms at 60deg and 200 Ohms at 90deg (ish)
 
#4 ·
If the coolant is only getting to 60deg it should be easy to feel that the radiator hoses are not hot, 90deg is the normal temperature so the difference should be easy to feel.
 
#7 ·
Right, I'm now 99% sure it's the thermostat, here's why:

I rest the MIL last night and all was fine when I got in the car this morning, drove around 15 miles and all was fine, then the MIL came on. I pulled over immediately and got out my fault code reader. Sure enough it was the same P0115 code as before, I checked the live data readings the engine coolant temperature (ECT) was 38 degrees and the temp gauge in the car was at the bottom. I left the car idling for 5 minutes and the ECT climbed to 60 degrees, I reset the MIL. At this point the interior gauge started to register 55-60 so that works.

I set off on my journey again but this time with the live data feed running on my reader and after about 1/2 mile the ECT had dropped from 60 degrees to 44 degrees. I carried on, then when I got another mile down the road the MIL came on when the ECT hit exactly 39 degrees. I carried on and reset the light and it only came on again when the temp dropped to 39 degrees again.

I got to my destination after another 10 miles and left the engine idling to pick up temperature and reset the light.

So my conclusion is, the thermostat is certainly broken, probably stuck open so it's permanently cooling the engine and not letting it heat up properly. Also the MIL is triggered when the ECT falls to below 40 degrees while the car is running/in motion.

Ed
 
#8 ·
odd about the MCSF (I presume to come on the engine must have got above 40deg first) ... but def new stat with those readings :thumbs:
 
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