o2 sensors, again

[jjgms]

My son's 2000 Corolla is getting relatively poorer gas mileage. No check engine light, but I've decided (after reading the threads) to change O2 sensor. Should I change both or just the one neat the catalytic converter? I live in Pa. if that makes any difference re: the post cat O2 sensor

[teppista881]

The pre-cat sensor is the only one that would affect fuel mileage. The post-cat sensor measures the efficiency of the cat itself.

[leeor_net]

Also check the MAF sensor -- a dirty MAF will kill fuel economy. They can be cleaned very easily with MAF cleaner you can pick up at any parts store. I would try that before replacing an O2 sensor as they're not cheap.

[phantazn]

How old is your son? When I was a teenager I got craptastic mileage with my 96 corolla, and it sure wasn't anything wrong with the car.

Also, make sure the tires are inflated properly, trans fluid isn't burnt up, oil isn't black sludge, etc. A downstream o2 sensor malfunctioning will not affect gas mileage. With how finicky Toyota's calibrations are, you will definitely throw a cel if something is amiss with either sensor.

[jjgms]

Took the plunge and ordered a new O2 sensor, pcv vlave and maf sensor. None of these have been changed since we've owned the car (10 years). It has 88,000 miles. Now all I have to do is find a diagram showing all the locations.
Next up, plug and sparkplug wire change, trans. fluid change

[WyRolla]

Unless it's actually physically broken, the maf sensor just needs to be cleaned, use maf cleaner or non-chlorinated brake cleaner, the pcv is only $6 or so, but can be cleaned as well.

I thought about replacing the O2 sensor on mine, but then realized that it's a huge waste of money if it's not throwing a code.