Oxygen sensor

[RyanM]

What symptoms will an old or failing oxygen sensor give?

[Pishposh30]

I don't know if age effects them but a failing sensor can result in the check engine light coming on, my mechanic also said lower gas mileage could result. This was when they suggested I replace one, which was failing interminably, after the check engine light went on. I decided not to spend the ~$300 and my mileage is still around 30mpg and the check engine light hasn't come back on.

[mazdaverx7]

thats true about the lower fuel mileage. it also depends on how you drive and if the ecu is recieving the signals it needs and accordingly adjusting the mixture.

[trioptimum]

My gas mileage was around 14mpg, after changing the downstream o2 sensor (on the exhaust near the drivers seat under the car), I got 22mpg afterwards. I spent $60 for a universal Bosch o2 sensor instead of $170 for a Toyota one and spliced the wires, works perfect. Dealer price for any sensor is around $300 (1hr labor). There are 2 sensors up front in the engine bay and the one downstream one.

The front two are pretty easy to DIY, the downstream one I put the car on ramps to do, pretty easy as well, but it was stuck on pretty good. Had to take out the drivers seat to get to the wiring harness.

BTW, get a haynes manual for your car .

You'll get a CEL intermittently, and poor gas mileage; sometimes the exhaust will smell like gasoline (runs rich).

[ShawnM]

You sure about that gas milage? The downstream sensors only job is to monitor the converters effiency. The upstream sensors are the ones that the computer looks at to adjust fuel.

[trioptimum]

Ehh, my gas mileage went up considerably when I did hte rear one. It was running really rich before. I did run Seafoam through it too at the same time, that could've done it .

[Axel Kain]

it will idle rough, and if you watch the exhaust, it may have a few little black puffs (running rich) otherwise its lean.
oh and it will affect the mileage.