i found this old topic, having very similar symptoms on my 2000 s4:

- check engine light on/off time to time
- gas mileage variability (actually pretty good)
- won't start sometimes. have to let it sit and come back and it'll start. has happened 3 times now.

i am going to look into the coolant temperature sensor, but i'm curious as to how this bad sensor would keep the car from starting.

also, i'm probably way more hands off when it comes to this than pretty much everyone else on this board, but if this is a simple enough fix i'd like to give it a shot. anyone have a resource i can reference for how to find the sensor and change it out? how about where to buy the replacement?

cheers.

 

You can buy the replacement from ECS, and its pretty easy. its in front of the battery compartment. you'll have to remove some hoses, and its tight down there!