So I have an 01' A4 1.8 T...

It failed emissions and the code given said it was running too lean and also stated the O2 sensor failed.... My tempurature warning gauge goes off whenever I drive over 45 mph, I have to run my heater on high and full blast to regulate it. There is no shaking while driving or stalling that occurs so it's making me question if the O2 sensor failed because of something else. I have notice that if I don't run my heater on full blast the tempurature guage starts to read on the hot side but the sensor itself does not go off, it only seems to go off then I drive over 45 mph. I was just going to buy and install an O2 sensor but the issues I'm having don't seem to have the symptoms of that type of issue. Help!

 

Does the warning go off after you drive over 45 regardless of how long you've been driving the car? As in, if you've been driving it for an hour around the city (i.e. below 45) then you get on the highway it goes off? As well as it goes off if you immediately get up to 45 without it being on for more than 2 minutes?

I'm trying to get an idea if the car is actually overheating or there's some connection the sensor and the speedometer.

Regardless it does seem like the car overheats. Do you know the last time the thermostat was changed?

As far as I know (and I'm often wrong) the o2 sensor doesn't have a lot to do with the coolant temp. So my initial guess is that there's two things up with the car: you need an o2 sensor replacement and you need a thermostat replacement. Since you're trying to pass emissions I would definitely do the o2 sensor first, then (and as long as you keep the temps under control) worry about the thermostat.