Hi everyone I just bought an 99 4Runner 4x4 V6 auto and Im having a very poor gas rendition so I started by replacing spark plugs,Air filter,Oil change etc etc but when i went down to replace my o2 sensors I only found one the up stream is this normal in a 99 4Runner?? please any advise

Yeah, that's normal for a non cali 4runner. One in the left header and one before the cat. Cali have a third I believe though.

usually you have one before the cat, to measure engine performance, and one after to measure the performance of the cat, right?

also i think Cali ones have two cats, an 02 sensor and an A/F sensor (instead of two 02 sensors)

That's the case on every federal emissions vehicle I've owned, and I'm pretty sure it's an EPA requirement.

Maybe Toyota did something different, but usually there's the upstream sensor to allow the ECU to adjust A/F ratio, and the downstream sensor to ensure the converter is working properly. I didn't know it was possible to do it any other way. The CA compliant vehicles have an extra converter, and more accurate sensors, but there's still just two of them. One upstream and one down.

I'd suspect this 4Runner has had some exhaust work and one was removed.

OP, do you get a check engine light? If not, does it light up for a few seconds when you start the truck? (Just to be sure the bulb is there and working).

Does the mid pipe look like this

If so you have a cali car and need a wideband (a/f) infront of the cat and a regular o2 after the second cat.

non CA vehicles have two regular o2's and one cat like this

for OBDII vehicles, yes. OBDI requires just a precat 02 sensor. and if you use some thing like EFIlive you can disable the downstream sensor on OBDII since it basically has no affect on how the ECM manages AFR

OBDII became mandatory in '95 I think, so his vehicle should have two sensors on it. So if the second one isn't there at all there should be a CEL, or it's been removed or disabled.

yup, it deff should be there somewhere, but that one wouldnt be the one affecting gas mileage even if it was deleted.

All 3G 4Runners are OBDII and have 2 sensors, before and after the cat or cats. If the second one is missing, and no P0420 or P0141 codes are set, then odds are there is a simulator box somewhere wired in to fool the ECM into thinking there is a working downstream sensor.

If your '99 is a Cali., it will have two cats and an A/F sensor before the first cat, but both 49-state and Cali. will have a downstream O2 sensor after the cat or cats.

Very true. If the CEL has been removed or is burnt it could have a bad upstream sensor (or any number of other issues) that aren't showing themselves.

Figuring out what codes are actually stored would be my next step.

This. It seems like O2 simulators are the "quick fix" for emission codes because the catalytic converters are stupid expensive for some reason to replace.

they contain platinum and other precious metals.