My car began feeling sluggish a few weeks ago, then generated a P0112 (IAT sensor low) and P0112B (engine coolant temp plausibility). ECT was only getting up to 80 degrees F on the highway, so I assumed the thermostat was faulty.

That didn't seem to explain the IAT code, though. The sensor seemed to be working normally according to my scan tool, but I pulled apart the hot side charge piping and cleaned everything up, just to be sure (no change). I use Alldata at work, but couldn't find any test procedures for the IAT sensor anywhere.

I swapped in a new thermostat housing this afternoon, which cleared up both codes as well as the driveability issue. Apparently, the low engine temp was tripping the IAT sensor code as well (I'd say this was strange, but we're talking about Mini here).

I couldn't believe how easy the thermostat housing is to replace, given the way people on here talk about them. Seriously, once the air box and zip tube are out of the way, it's like 3 bolts, some hoses, and a clip. Had I not opted to flush the coolant, I could have had it done in about an hour. Don't be intimidated by this repair.

The thermostat housing is about $150 from Mini, and you will need one gallon of Zerex G-05 coolant which runs about $18.