A mechanic replaced BMW X3 2008 cracked coolant reservoir. Car was running fine for several months. Recently "low coolant level" light came back on. I refilled it with more coolant. However, now it leaks coolant very quickly and there are even coolant leaks visible below the car.

I removed the air filter hose to see where the leak could be coming from and fished out this sensor(?) harness that obviously was unplugged even before I got my hands on the car:

Some of the harnesse's housing plastic appears to be grinded off by touching accessory drive belt

What I am trying to understand is -

  1. What is this harness?
  2. Could it be that, if this harness is not properly connected, then it can trigger overpressurized condition in cooling system and cause a new leak (the new leak seems to be coming from water pump area since it is the wettest one, but I really can't tell for sure yet)?
  3. I haven't cheked, if the reservoir cap is working properly and can release excessive pressure, but it appears that around the bleeder screw there is coolant build up. Is BMW Coolant capable of sealing small leaks? Could it be that bleeder screw for these several months was the only place where excessive pressure was released and once coolant sealed it the system got overpressurized and caused the new leak?
  • I would take it back to the mechanic.
    – Moab
    Apr 29, 2018 at 1:28

1 Answer 1


That damaged, plastic piece is Air Quality sensor and probably would not cause overheating on its own.

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