That is correct. There are 2 on the 98 4runner. Toyota can hook up their scanner with the car running and monitor each O2 individually. One was barely working and the other was intermittently working.
I had a bad O2 sensor on my 98 4runner. My MPG went from 18+ to 9 MPG. The engine will run rich, hence the bad MPG. This did not throw my CEL because the O2 sensor intermittently worked. I don't remember how much it cost but the $300 range sounds familiar (dealer installed).
I just filled up and my Range indicator estimated 476. I filled up with 465 miles on my tank with 2 gallons left. Came out to be a little over 22 mpg (computer said 24.3 mpg). I have 05 V6 4X4 running 85 octane. I reset my MPG at each fill up.
I routinely get between 21 and 22 mpg in my 05 4runner 4X4 V6 without AC turned on. My driving is mostly highway and my city driving is on roads with 40-45mph speed limit. I usually fill up with 425 miles on my tank with the gas light just coming on. Computer will show 30-40 miles left on...
I would bet that your oxygen sensor is bad. This happened on my 98 4runner. While I normally got 18-20 mpg I was suddenly getting 9mpg. A bad oxygen sensor will cause your engine to run rich, hence more gas.
