Thread: Up for debate p0420 code and bad gas mileage..

I am debating with my buddy about, Converters and oxygen sensors, he has a 02 prix non supercharged 3800. He bought it from a younger kid and the car had a bad maf sensor, so no telling how long the previous owner drove it like that. So a bad cat is totally possible.


He changed the maf ,and the gas mileage went from 11.6 to about 11.8 miles a gallon and he has a PO420 code. I know that code can mean many different things. But The debate from my stand point on these cars the only time a cat can cause bad fuel mileage if its clogged. I don't think the rear o2 sensor does anything but read what the cats doing, It doesn't change fuel trims etc.. But I could be wrong of course.

We did the the restriction/back pressure tests (the easier ones) with the gauge hooked to the intake manifold. Not sure if those are good enough and they all can back with normal readings.
I suggested he do a tune up first, car has 99,000 on it.

Oh ok well , I guess repairing that problem is still up in the air as far as oxygen sensor and or converter.

Does the car run krappy tho..?? Seems like it would have to geting such low mpg...

Sometimes when he gets up to around 20 mph the engine will stumble. It does not do this consecutively. Besides that it seems to run fine. But its definitely something going on, only getting around 12 mpg.

Thanks guys, he has driven it the last few days with no stumble.. I would think if it was the cat it would stumble everytime? But I guess your saying the cat could be bad, but not clogged though.


We cleaned the idle air control and mpg went from 11.8 to 12.2 ....



What are some good brand cats to buy??

is the CAT on the car now stock or hi-flow?

The car has not stumbled in a few days.. I think it was related to the maf sensor.

But the cat is Stock.

The fuel used/mpg gauge on the dash....If that's even accurate.

oh ok, I'll keep that in mind thanks.