Hey guys, Im back in the game. Picked up a 1990 4Runner 3.0 4wd today for $2500. Runs great except for an accelaration problem. Engine code is showing 21 which Ive done my research and found that its an oxygen sensor. So I called up my local Advance Auto and did a price query. Then I was asked if it was a upstream or downstream sensor. I felt like a fool because I have no idea. So my question is: Is a code 21 an upstream or downstream oxygen sensor? Please dont bash me, I tried to research this without having to do this. Thanks in advance for any help. I will post pictures as soon as I can take them, its raining today.

 

I might be wrong here (I have had a beer or three) but I think the "upstream-downstream" thing refers to th O2 sensor being before or after the cat.

Let me do some looking and I'll get back to you.

 

Yep. Upstream=before the cat and downstream=after the cat. If your truck is not a cali truck, it should only have an upstream sensor.

 

the 1990 3vze 4runners I have seen here in california only have one sensor between the cat and engine not one after the cat.
Not really sure what year they started having 2 sensors.

 

All 4runenrs and trucks sold in CA should have the two sensors plus a bunch of other sensors and emissions crap on them. But that wouldn't stop someone from buying a used runner from some other state and bringing it to cali, that's probably why the runner you saw had only one sensor.

 

My old '91 3.0 longbed had only the 1 o2 sensor before the cat. My '91 Wa 4Runner has 1 o2 sensor as well. My FSM only shows 1 o2 sensor, before the cat.

 

Thanks for the help guys. I only saw one underneath so I guess that narrows it down to the upstream sensor. Thats why I love you guys more than my own family!

 

Do yourself a favor & get a replacement Denso or NGK (NTK for O2 sensors) at www.sparkplugs.com. DO NOT get a bosch...

 

If it were a downstream o2... you would have a code 27....

 

X2! Our rigs don't play well with Bosch stuff......

 

Thankx for the Bosch tip, thats what I was going to go with cause it was the only thing in stock. But I havent put it on yet!

 

Ya dont use the bosch. It might work for a few weeks but WILL crap out on you. It sucks to pay so much more at the dealer but in the end there is no comparison.