Just swapped out the water pump, thermostat, and thermostat housing on my 2010 550i. Now, after driving 15 minutes or so, I'm getting a check engine light, and my code reader says it's a P0128, coolant temperature below threshold. If I use the hidden menu on the dash, it tells me that the coolant temp after driving 15 miles or so on the highway is 92C, which seems to be exactly where it should be. I assume since the vehicle can read the coolant temp that the coolant temp sensor I transferred over from the old thermostat housing is working....

Any thoughts on what could be causing this code? I wasn't having this issue before swapping out the failing water pump.

 

IIRC, that code is set if the coolant temperature doesn't reach a target value in a certain amount of time (maybe less than the 15 minutes in your info above?). I'd suggest finding out what the proper criteria for a "passing grade" is and checking to see if that's happening. If not, I'd suspect that thermostat.

On a related note - what brand is the thermostat, and where did you get it?

 

Issue appears to have resolved itself....I'm thinking I might have had some air in the system. Swapped out the coolant temp sensor just to be safe.