I've lost MPG and I beginning to think it is my 02's. I replaced plugs and wires, and had the fuel injection system professionally cleaned. The problem is still there. It seems like when I get on the truck hard I can watch the average MPG go down 1-2 tenths. Then the truck feels like it is running lean. Would the rear 02's be doing this if they were wearing out and not adjusting fast enough. Feels like its running pig rich IMO. I have 02 sims on the way.(bought them off e-bay) Do the rear 02's control this or would it be the upstream/front sensors? Other than buying new sensors or spending $100 for a diagnostic test, is there a easy way to test?

 

Rear O2's from what I have heard just monitor the cats, they dont actually adjust the engine. when the cats (or the 02's) go bad it throws a code. Are you gettin a code, if not you can test them with a voltmeter to see if there still good.

edit: oh ya, autozone does free scans at there stores......

 

Correct, rear O2's only set the SES light to tell you your cats are not working. THey have nothing to do with air/fuel mixture.

My O2 sensors started to get lazy (slow to respond) around 75K. I dropped about 1.5 mpg over the year prior to replacement. I was not setting any codes but replaced them anyhow. I went with Bosch specific to my vhehicle (not universal ones) and my mileage IMMEDIATELY came back. They were not cheap ($80 ea). The Bosch have been going strong now for over 80K so I'm hoping I don't need to replace them again for a while.

BTW-The voltmeter will give you a voltage but it will not respond quick enough to verify proper operation. You can test if it has failed completely but you cannot fully verify operation with a meter.

 

That sounds like my truck for sure. They are definately slow to respond. I'll be getting two new 02's soon. Thanks for the responce guys.