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I thought I would create this post in the hope it might help someone else who has a similar cooling system issue with a 2006 Mini Cooper R50. The one we have is now almost 14 years old and covered 80K miles. It is UK spec and has the chrono pack so the engine temperature gauge is in the centre console where the speedo normally is, along with the oil temp and oil pressure gauges. We have owned it for over 10 years now, and for the last 7 it has been mostly owner maintained. Generally it has been very reliable save the usual wear and tear items. Recently though I noticed the coolant level was dropping in the header tank. On closer inspection I found that the radiator had started to leak, which is not too unexpected on a car this age. A new radiator was fitted and after running the engine up to normal temperature I kept wondering when the cooling fan would cut in, the temp gauge was hovering around the 12 o'clock mark for ages, then it went just past and it started to boil over! After doing a bit of research I found that the cooling fan did seem to work when the aircon was switched on, but not when it was off. Further research and investigation revealed the cooling fan stage 1 resistor had failed which I duly replaced with an aftermarket resistor. However the fan still did not cut in when expected! Further testing showed that both stage 1 and 2 fan relays were working so attention turned to the DME and engine temperature sensor. Searching on this forum I found a test which involves disconnecting the temperature sensor connector which should make the DME energise both fan relays. I tried this and sure enough both relays kicked in and the temperature gauge showed maximum (120º) so that gave me confidence that the DME was probably OK and the issue lay with the temperature sensor. So I then replaced this with a genuine BMW part and was glad to find the stage 1 relay now cut in as expected when the temperature gauge went just beyond the 12 o'clock mark. However one thing to note about testing it in this way is the DME now registered a fault code which had to be cleared with a fault code reader/reset device. What surprised me about this issue is the original temperature sensor did not appear to be obviously faulty, the temperature gauge seemed to be behaving as expected by quite quickly reaching the 12 o'clock (normal) position but in hindsight was not showing a high enough reading when the car boiled, after I fixed this I found another post on here that states the stage 1 relay should cut in at 105C and the stage 2 at 112C. Had I seen this earlier or been able to find it elsewhere, like in the Haynes manual, it might have made me realise the temperature sensor was faulty sooner (although the gauge itself is pretty basic so it is difficult to tell what 12 o'clock is supposed to represent in terms of temperature!). I just hope this helps someone else sort out the same issue if it happens to them…
 
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