I didn't notice any change in mileage after I changed mine.

 

How about the oxygen sensor?

 

Q: Which sensor(s) affect gas mileage, post or pre-cat? I've heard its only the pre-cat, but why would the ECU ignore the post cat reading?

 

The pre cat O2 sensor does all the fuel trim calculations, the after cat just reports the efficiency of the cat converter, thats all.

 

When you say, poor fuel economy - what sort of mileage are you talking about?

I just want to be sure that I'm getting the right sort of returns from my engine. My 2003 A4 1.8T Quattro has 47K miles on it ... and is giving me about 23 miles on average (city + hwy together ... on a long trip, I get about 27 mpg).

Your thoughts would be appreciated!

 

Shaf, that actually seems normal. On the wife's 02 1.8TQ we get around 20 around town and 27 on the hiway (between 70-80mph).

 

Where is the ECT on the 1.8t?
Mine says Circuit low input..maybe a fuse, or the sensor its self?