P2181 after thermostat install: Coolant Temp Sensor or Thermostat issue?

[natemail]

I have had a coolant leak and a P2181 code recently. Checked the leak, coming from water pump. No problem, I replaced the water pump AND thermostat last weekend. STILL getting the p2181 and the coolant temp gauge is not getting up to the middle line, just like prior to the replacement of the water pump and thermostat. Check VCDS under IDE04083 (Coolant Temperatures) it reads: ETC 1: 65C, ETC 2: 98C.
Seems to me like either the thermostat I just installed has already failed, or that my one coolant temperature sensor has failed. Since I was getting the code with both the old and the new thermostat, that would lead me to lean towards the sensor.

Can anyone pitch in as to how I can test the CTS? Is there a mistake I could have made installing the new thermostat that is causing it to stick open? Every thread I read seems to point to thermostat but I just replaced that part. A bad CTS right there on the front of the engine would be much easier to replace. The part I installed in my 2013 S4 was the Rein CTA0017 which I purchased from FCPeuro.

[LowKeyLoki]

It’s more likely you have air trapped in the system still. Did you vacuum bleed the system after replacing everything?

[natemail]

It’s more likely you have air trapped in the system still. Did you vacuum bleed the system after replacing everything?

I also wondered this, my vacuum bleeder actually arrived today. Can I pull vacuum while the car is full of coolant and add from there? Or do I need to drain the whole thing and start from empty?

[natemail]

Turns out it was just the CTS (coolant temperature sensor) for me! Normally sensors don't seem to fail by just reading 30deg C low, but mine did. Swapped the sensor in five min with hot coolant (not recommended) and started the car, with the new sensor, gauge went right to where it should be. VCDS has the two sensors now reading less than 5deg C apart. Love a nice easy fix.

[TC_S4]

I also wondered this, my vacuum bleeder actually arrived today. Can I pull vacuum while the car is full of coolant and add from there? Or do I need to drain the whole thing and start from empty?

Drain the whole thing and start from empty is the recommended way to avoid any air bubble on the system. What I did, is suck out as many coolant from the reservoir and start the vacuum. When the psi reach it max and holds the pression for at least 15 second, start filling. To be sure that there are no air bubbles trapped, I crack the SC bleeder bolts, never did the heat core, the IAT is good, heat are strong, so didn't bother.