Where is the oxygen sensor ??

I just got a 1990 1500, and it uses way too much gas. I was told that it coould be the oxygen sensor. I did the self test on the the truck and code 13 popped up, it is the code for the oxygen sensor. According to the book that I got, it is supposed to be in the pipe behind the manifold, but I can't find it. I don't want to sound stupid but I am lost. Can anyone help me out? My first thought was that maybe the pipe was replaced and the sensor was never replaced, but I don't even see a loose wire down there. Any info would be great. Thanks

The sensor should be on the passenger side just after the manifold/Y-pipe flange.
If the sensor isn't there, you would have a very bad exhast leak. That is unless someone has replaced the exhaust pipe.
The truck would run without the sensor but it would screw up the computer and could cause all kinds of problems.

O2 sensor is on the driver side header just before the header to y-pipe flange. It should have 1 purple wire coming off of it. If you are looking for decent gas mileage--get a different car/truck. I have an 89 1500 and get 12 mpg if i am lucky, 14 on the highway with a tailwind, these things suck gas like no other.

the O2 sensor is on the driver side . it is right thier you cant miss it. my 1500 O2 sensor is very easy to see