The coolant temp sender has failed if the coolant temp gauge in the instrument cluster lives up to VW's parts reputation (Varies Widely). If the gauge informs you that the engine is stone cold while you're running 85 mph, it's time to replace the sender.
Part Information & Sources:
Note - always confirm part numbers by looking at the actual part mounted in the car! Mistakes will happen sometime, somewhere.
1.8t VIN: pre 3B-X-491580 - 078 919 501 B (blue)
1.8t VIN: post 3B-X-491581 - 059 919 501 A (green)
OEM Pricing: ~$20 (blue)
OEM Pricing: ~$6 (green)
Impex - $5 (green)
The sender is located at the rear of the engine, and is one of the few easily accessible parts on the car. (illustration)
Note that the coolant system is sealed, and thanks to the vertical orientation of the sender, the coolant will pretty much stay where it should when you pull the sender (no need to drain coolant!).
- Start by pulling the electrical connector from the sender. (illustration)
- Pry the retaining clip out. (illustration)
- Pull the sender directly upwards to keep from distorting the o-ring below. (illustration)
A small amount of coolant will flow out, just stuff some paper towels under the sender plug to catch it. If coolant flows freely...you have a bigger problem to worry about than a bit of coolant on the ground. :)
- Take a look at the offending component... (illustration)
- Pop in the new sender, making sure it's seated well in the o-ring. As long as you don't abuse the o-ring there shouldn't be a reason to replace it. If the o-ring is deteriorated for some reason, you can pick one up from the dealer (part #: N 903 168 02) - but if the seal is deteriorated you need to find out why, as this will indicate deterioration of all rubber components of the cooling system! Start crying now.
- Snap the retaining clip back in place and reconnect the electrical connector. If the contacts for the connector look dirty, use an electrical contact cleaner on it.
Done! Run the engine, check for leaks, then go for a drive...