I have a 2000 4-Runner with a check engine light, Toyota said it was reporting a "lean" mixture and told me that it was because of the brand of gas I was running, or a bad O2 sensor.

1 - how many o2 sensors are on a 2000 4-Runner, there are, what looks like 2 - one before the cat and one after?!?!

2 - if there is only 1, which one is it, front or back?

Thanks
Tim

Welcome to the AF!

What code did the ECM report? Other things can cause a lean error code besides the HO2/A-F Ratio sensor.

(There are two sensors, one before and one after the cat. converter)

Welcome to the AF!

What code did the ECM report? Other things can cause a lean error code besides the HO2/A-F Ratio sensor.

(There are two sensors, one before and one after the cat. converter)

I didn't get to see the code from the dealer, and the one at the local autoparts store has a broken pin right now. I will get the code read tomorrow and I hope it narrows things down a little.

With there being two sensors, should I replace them both at the same time?

No, they have different functions.

I bet the code is P0171.

Read this thread:
http://www.automotiveforums.com/vbulletin/showthread.php?t=405771

If it turns out to be an HO2 sensor, then make sure you get the right one. Some trucks have the HO2 sensor and some have the A/F ratio sensor. Get the correct part number from Toyota parts department with your VIN.