Have Volkswagen Questions? Ask an Expert Now.
The actual problem here is the thermostat- it is sticking open just enough to allow a small trickle of coolant through, so the engine doesn't reach operating temp in a mapped amount of time and sets the fault.
The thermostat housing is behind the alternator- if you take the engine cover off you can see with a flashlight that it is a black plastic coolant flange held on with 2 10mm head bolts.
If you are careful, you can use a ratchet with a long extension and a swivel 10mm socket to get to this- just have one hand in there to try to catch the bolts so they do not fall, use a magnetic socket, or use a magnet on a stick to grab it. DO NOT TOUCH THE TERMINAL ON THE ALTERNATOR or disconnect the battery so you don't risk it. IT WILL ARC.
When tightening the housing back up, make sure the bolts are in place, then hold the housing in place so the gasket does not move, and tighten the bolts finger tight on each side before fully tightening, so it doesnt kink to one side and make the gasket pop out of place.