2001 tacoma 3.4l oxygen sensor wiring diagram
 

Got an oxygen sensor heater high/low code for the upstream sensor that I want to investigate before I buy a new $140 oxygen sensor. I need to know which two wires are for the heater so I can do some tests with a multimeter.

the two black wires

And don't spend $140 for an A/F sensor (what the upstream exhaust sensor actually is). Go here: http://densoautoparts.com/find-my-pa...icle-selection to get the OEM Denso part number for your sensor. Then go to Amazon and get the IDENTICAL sensor in a Denso box for much less. You can Google the part number to check prices elsewhere and it's good to check rockauto.com and partsgeek.com, but Amazon usually has the best price.

Should be this one ($103 Prime eligible):