My 1994 2.4 liter 2 wheel drive pick up, which I drive very infrequently, began getting louder several months ago and a few days ago began making much louder noise from exhaust system (some significant leak presumably). I had the exhaust manifold and the adjacent exhaust pipe replaced almost 2 years ago. When the noise got louder, like the muffler was no longer working, I brought in to the shop that replaced the exhaust manifold. They gave it a quick look and when we opened the hood you could easily see where there were two empty bolt holes on the manifold - the obvious source of the noise. They said bring it back in and they could repair but could not tell what was missing.
For convenience, the next day I brought to a toyota dealer (who has been good to me) and they said that the manifold was missing an oxygen sensor. BUT that the truck did not originally come with an oxygen sensor and it did not connect to anything. They implied that an incorrect manifold was put on.
I called the original shop back to ask about it and he said it did need an oxygen sensor and he looked it up again and said that's what his source was showing.
So I am confused. Does anyone know definitively if this model of truck, should have an oxygen sensor on the manifold?
Thanks.

 

A picture would help. A 94 has the O2 sensor in the exhaust pipe and not in the manifold. On 87 and older trucks the O2 sensor is located in the manifold. From the way I am understanding your post, it sounds like someone has used an 87 or older manifold on your truck for a replacement and the old sensor or the block off plate has come off.

If your truck is a 94, the correct manifold will not have the cut out for the O2 sensor. You can see where it could be cut out to make an older manifold if need be. I have used manifold made by Dorman and have found them on ebay or I think your local part stores can order you one.

Basically there are two different manifolds for the 22re motor, just depends on where the O2 sensor is located, either in the manifold or the exhaust pipe. In a pinch you can use the manifold that is on your truck, just need to stick in a single wire sensor or block it off.

California models have two sensors, in front (upstream) and in back (down stream) sensors.

Depending on if your truck, some new 22re trucks had an O2 sensor in front and in some cases behind the muffler. If your truck is fuel injected, it did come with an O2 sensor. (I think 92 was last year of carbureted trucks.) and it will be located in the exhaust pipe under the drivers seat area. You will need the 4 wire sensor. I get mine from www.sparkplugs.com get the original and exact fit sensor. Stay away from the generic, fit all sensors. Denso is what you need to get for a replacement. No Bosch brands.