Coolant Circulation Pump
Coolant Circulation Pump A31m1
Acknowledgement of contribution by BenzWorld Member, Pokseva.
Issue with Coolant Circulation Pump
It may be hard to notice that your "coolant circulation pump" (also known as - "Auxiliary Water Pump", "0 392 020 066" or 0392020066, A2208350064, A31m1) is not working properly.
I have finally found that my pump was dead, and in cold weather my car could not be heated more than 30C even if AC was set to "HI".
Diagnosis:
When reading error codes with STAR I got this error code: "B1416 The component or the signal line to the component A31m1 (Coolant circulation pump) has Short circuit to positive or Overload."
As it turns out B1416 error code is thrown if coolant circulation pump draws >5A current.
Testing:
I removed the pump and connected it to 12V DC. The pump did not spin at all (it was completely stuck and I could not rotate it even by hand) and the multimeter showed that it consumed 5.5A current.
Conclusion:
While you can buy a brand new pump for 140 EUR you might as well try to repair it. The reference shows how you can do it.