i picked up a v6 3.4 with a gutted cat the gas mileage is terrible (somewhere between 10-15 mpg) i took it to a muffler shop and i was told that the o2 sensor was getting a false reading because the cat was gutted is this true and what can i do to test/fix it? i get a ses light on the highway as soon as i let off the gas but for some reason he couldnt get a reading from the computer any help or ideas is much appreciated

 

Is the O2 sensor behind the cat? If not then the O2 shouldn't throw any codes. I would start with a good tune up (plugs, wires, filters) and see where your at.

 

The O2 sensor on a 3.4L is before the cat so that should not be your problem. Check plugs, wires, coils, fuel filter, egr valve, pvc valve, air filter, etc.

 

Pre-cat: will affect MPGs

Post-cat: won't affect MPGs, simply checks cat efficiency.

o2s are cheap enough, replace it and see if it helps. Won't hurt having a fresh o2 on there.