Car: 2004 Audi A4 1.8T FWD 4Cyl. 90,000 miles

The cars temperature is normal ( at half way mark ) but once the A/C cold is turned on, the engine temp. gauge shows a significant drop, to the 1/4 mark.. stays that wait til I turn the cold off, then starts going back up to the half way mark. ( started happening around 70,000 miles )

My check engine light is on. Two codes:
P2181- Cooling System Performance
P0118- Engine coolant temp circuit high input.

I was told that this is normal because when the AC cold kicks in so do the cooling fans which cool the engine and that is why I see the significant temp. drop on the gauge.

But at the same time I am thinking the thermostat is not operating 100% because the engine temp. should be at the half way mark AT ALL TIMES.
Another problem might be the engine cooling sensor.

 

I had this on my old a4, turned out to be a combo of thermostat and coolant sensor. I'm sure the sensor was fine in my case, but I replaced it for piece of mind since it was only 25 or so dollars. That gauge should never move from 1/2 unless its shut off and cooling down or cold start to a warm up.

 

Yeah that's what I will replace too since I am at the 90k mark I am doing my timing belt/water pump/serpentene belt service so might as well do the t stat while in there.

Another thing that I find weird and this is sometimes, not all the times... when I have my ac cold on , it briefly turns off for a second and then kicks back in... i hope my compressor isn't giving out..

I am hoping this whole tune up I am doing will fix everything because my car has been driving sluggish every time I use the AC.