hey guys My 1997 A4 1.8T is getting 13mpg in the city and 19 on the freeway. i logged the sensors and the pre cat sensor responded pretty good but the post cat seemed a little sluggish. From waht i hear is the pre cat sensor is the important sensor. so would the post cat sensor cause this much damage. thank you

 

I don't think the post cat oxygen sensor does much. It wouldn't be messing with your gas milage. If anything you would want to replace your pre-cat.

 

basically all your second o2 sensor does is tell the ecu that your cat is doing its job.....( for those of us that have one ) so i dont imagine your o2 sensors would have to do w/ this, possibly an injector or maybe a spark plug.

 

cat problem, basically the first o2 sensor ( that's the one closest to the exhaust manifold ) is measure emissions the second one compares the results to inform the ECU of the cat performance,

when both o2 sensors reach a similar reading then this points to the cat being the problem rather than anything else, as your mileage is that low I would say the cat is bad

 

their is nothing wrong with the cat. I had the cat checked out. If i log the oxygen sensor what should they read? i am thinking of just changint the pre cat sensor because it does have over 100000 miles on it.

 

Just change the pre-cat sensor. I don't know if anyone knows what the exact readings should be, I sure as hell don't study every reading I get from my car especially from an emissions related device.

Depending on your engine code, the sensor could range from $40-240.

 

Air filter or MAF or both.............

 

front 02 should cycle above and below .45v. The post cat o2 should stay around.8v.

 

like you, I too have a '97 A4 that had a similar problem...The MAF was the culprit this time...might wanna check it out...

 

Also, all the stations have now phased out MTBE as an additive and have started using 10% ethanol as an oxygenating agent. Most cars are experiencing a drop in MPG with the changeover.