I purchased this van in 1998. Just last week I get code 51 after 17 years. Crawl up under it and there is no 02 sensors anywhere on the vehicle. So it was obviously built without an 02 sensor? Or is it integrated into another component? It does however have a smog pump system and EGR valve system. I replaced the throttle position sensor because it came apart in my hands while inspecting it, reset the computer and still code 51. I am experiencing the reduced power as in stuck in lean position. Map sensor? Please give me some ideas.

 

90 02 sensor should be on driver's side exhaust manifold accessible through wheel well.

 

Someone else off this site said passenger side below the manifold and wire plugs into a black box in the fender well. I'll look again tomorrow in daylight. I didn't find one 2 days ago.

 

Another local mechanic said it probably didn't have one since it has a smog pump.

 

Can we assume that is actually code P0051?

 

Well it's actually the old school (1990) turn the key 3 times and read the blinks. 12-51-55. 12 being the starts since the last time the battery was disconnected, 55 being the end of the fault codes list. I guess it's basically the same code. Just an older system.
I'm going to look a little harder in different locations to see if I actually missed finding the sensor. One said it could be under the manifold in the head. What do you think?

 

I'm starting to think it may be the computer, again. I've replaced it twice over the years.

 

Finally found it looking through the drivers side wheel well. Specialty tool time. Thanks to all who replied.

 

Landyacht got it right. Thank you so much. For all who may have the same problem. You really can't see it except through the wheel well on the drivers side. Very little tork room to remove it (just inches). Break off the ceremic top and use a 7/8 very tight socket set and a very long tight fitting cheater pipe to remove. Autozone rental tools are tapered. You'll never get it off with those. However installing will require it. The very smallest of the rental tool set is easy. There is a flange in the way so the larger ones is too think. You can't over tork it either.The tapered edge of the Autozone tool will slip off just as you get it just tight enough.