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Thread: Coolant Temp Sensor and other work

  1. #1
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    Default Coolant Temp Sensor and other work

    Got some work done on my 4.2 S5, changed the battery, fixed some minor leaks and changed the upper and lower control arms. Low and behold the fans stay on and flatten the battery while I'm at a meeting about an hour from home. RAC come to the rescue and jump start it and I roll back to my new favourite mechanic Euro Technik who spend a humber of hours trying to work out why the fans are staying on without any fault codes being registered. Well eventually a fault code turns up and it's for the coolant temp sensor which I then had repalced. Seems to have fixed the issue but I need to take it on a decent run to get some real heat into it to test it out. I've got one outstanding issue related to fuel trims that is also not causing a fault code but the two O2 sensors are show lean and the O2 sensors in the exhaust are showing rich. Looking at the amount of soot in the exhaust it is definately rich. I've looked at heaps of solutions and my new favourite shop haven't found anything so far other than eliminating, injectors and fuel pumps anyone have any other ideas?

  2. #2
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    Let it sit & idle for 20min.

    Fans should kick in as thereís no air flow through radiators to cool it down.


    Sent from my iPhone using Tapatalk

  3. #3
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    Looks like I still have 2 issues. Fan is coming on fine but then it stays on and flattens the battery, fuel trims are still showing rich I think it might be the evap purge valve allowing a small amount of unmetered air into the system. Fans are really annoying becuase it looks like the controller might be the culpirit but there are no error codes so can't categorically say that is the problem.

    Once you put the boot into it though the car is a joy to drive.

  4. #4
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    Default Coolant Temp Sensor and other work

    Quote Originally Posted by Iwant1 View Post
    Looks like I still have 2 issues. Fan is coming on fine but then it stays on and flattens the battery, fuel trims are still showing rich I think it might be the evap purge valve allowing a small amount of unmetered air into the system. Fans are really annoying becuase it looks like the controller might be the culpirit but there are no error codes so can't categorically say that is the problem.

    Once you put the boot into it though the car is a joy to drive.
    Are they short term or long term fuel trims?

    Unmetered air causing issue with primary 02 sensors in thinking car is lean therefore adding fuel hence now running rich?

    Stock or modified exhaust?

    Fan controller is a front bar off to replace and get access.

    The wiring harness for fan controller connects to a plug connector under the cover next to the power steering filler.

    Big 2 pin connector with 10guage wire you canít miss it.

    You could disconnect at connector as a simple test to see if it throws a code.


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    Last edited by mike; November 15th, 2021 at 10:15 PM.

  5. #5
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    Always good to put on that the issues are solved. Fan Controller was the culprit so had to replace the whole unit $$ good fro another 12 years. I'm not convinced that the fule trims are any issue atm so I'm letting it go unitl it throws a code. Everything is buttoned up tight now with no issues and it runs like a clock.

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