I'm driving a 2002 Galant DE. Had vacuum leak, cat efficiency, up 02 codes. Replaced throttle body gasket and intake hose/tube. (Fixed leak/ rough idle)

And replaced the ORIGINAL o2 sensor (118k miles... :/) with the same exact Denso o2 sensor, brand new. Was getting 25 mpg mixed before, looks to be about 15 or so now.

I have NOT done anything about the cat eff. sensor code. WITH that being said, any reasons why my MPG could have gotten WORSE after replacing the o2 sensor??

 

Did you replace the o2 sensor before or after the cat? Did you reset the ECU afterwards?

Perhaps the cat is a wee bit clogged. You can remove it and soak it in acetone/gasoline. That'll clear it up some. Just DO NOT put it back on till everything has completely evaporated off.

 

It was the upstream one before the cat, on the exhaust manifold. Changed o2 sensor in Advanced Auto parking lot, they reset ECU for me.

I haven't tried cleaning the cat, but it IS giving me a cat-code, so I'll try your suggestion and let you know.

 

Reset the ECU yourself, remove the negative terminal on the battery with the keys in the ignition. That'll drain any excess power.

Make sure you remove the rear o2 sensor before you dunk it, that'll destroy the sensor. I would suggest replacing your air filter too.

 

Now I'm not an expert, so I'm not like insulting you or anything, but how would the cat have anything to do with the MPG? Isn't that (*and down 02 sensor) just purifying the exhaust that's coming out of the tailpipe, (*and measuring the contents of the refuse).

A friend of mine said something about it clogging, but I don't fully trust him. (Right after saying that, he asked how to fix his window because it doesn't go up)

 

Because of the exhaust restriction, that's why it lowers fuel economy.

The o2 sensors directly effect fuel economy too.