Okay, I have the 2003 Quad Cab with the V6. I have a P0138 code which tells me that it's the downstream left bank (drivers side) O2 sensor. Problem is I believe I have 5 O2 sensors. 2 before the cats, 2 after the cats, and 1 in the exhause pipe between the collector and the muffler. Does that make sense? So which downstream to I look at the one that is right after the cat on the drivers side or the one way down by the muffler? Or am I completely wrong and what I'm seeing down by the muffler is not an O2 sensor? Haynes only shows the 2 before and 2 after the cats on each side.

I'm confused.

 

there is only 4 sensors.. the 5th thing could be somthing else.. i dont know.

But the code is talkingh about the o2 sensor AFTER the Cat...

upstream would be the sensors before the cat, and since you have this code, replace the o2.. if the code comes back, you might be looking into some new cats, or you have a spliced wire/bad pcm (these 2 are less likely to happen)

 

Okay, Took a very detailed look at my truck this morning, and I have only two O2 sensors. I have the V6 quadcab so maybe that's the difference?. I have one Cat, underneath the truck. There is definitely no cats off the exhaust manifolds, and the exhaust manifolds run into one pipe shortly after they both leave the engine compartment. There is one O2 sensor before the cat, and one directly after. It appears that the one directly after is the bad one.

 

you are el corrrrecto!