Hey guys,

Just bought this truck a few months back (96 xl 5.8l, e4od tranny). The exhaust was modified by the previous owner, and now it has true dual exhaust. It has what i originally thought was two cats (one on each pipe), but i suppose they may be mufflers (how would i tell the difference?)

Anyway, there are three o2 sensors that i can tell, one directly downstream of each each exhaust manifold, and one further downstream on one of the pipes, but upstream of the catalytic converter (muffler?) it's my understanding that this o2 sensor should be AFTER the cat.

What (if anything) should i expect from this? My CEL is on and i get an OBDII code p0171 (bank 1 lean). any connection? Should i rework the exhaust with the cats upstream of that 3rd o2 sensor?

Thanks!

 

It's a direct connection. The upstream (before cat) O2 sensor tells the engine if the mixture is lean or rich. The downstream (after cat) O2 sensor is just to check the cat for proper operation. For your CEL, you only have to worry about the first sensor you come to after the exhaust manifold (upstream). If one is bad, I would go ahead and change the other one out at the same time. If one goes out, the other probably isn't too far behind it. Keep in mind that your sensor may be good. You could truly have a lean condition on that bank. I would also check for an exhaust leak there before you go changing out the sensor(s).

 

Another problem is that the exhaust is clearly not the way the factory intended so the sensors may be in the wrong location. They are designed to be a specific distance from the engine. If they are too far you can get an incorrect reading. I am pretty sure that what you have are mufflers not cats. Few people put on a system like that and replace the cats and I've never heard of anyone running twin cats and no muffler. A photo might help. You can check with a good muffler shop and ask about proper placement of the sensor. The fuel system will never run right without it.

 

I'm sure the left and right are fine as far as location, but one is reading lean. The rear one should be behind the cat, which sounds like it was removed, and also will keep the check engine light on but not that code.

 

Replacing an old sensor is a good idea anyway. You could try that first. Like Sean said you will likely still get a code from the 3rd sensor but the first 2 are the important ones.

 

Ok, so today, a few new codes popped up. P0141, which from what i've read, might be the code you'd expect from a nonfunctioning (or in my case nonexistent) cat.

So for the p0171 code, would you guys recommend changing the bank 1 o2 sensor? I've checked for vacuum leaks and cleaned the MAF. The fact that it is reading lean only on one side only makes me think that either that particular o2 sensor is bad (though i should expect an o2 sensor specific code right?) or there is some bank 1 specific problem (fuel injector, manifold leak?)

Thanks!

 

The drivability isn't that bad with the truck, but under load, I do get what I think is a pinging and slight hesitation. Plus, I figure I am probably losing some gas milage if I am running lean, and I don't want to damage the engine, so I'd like to get this p0171 business resolved.

I was going to switch the O2 sensor from Bank1 (passenger side, reading lean obdII code p0171) to bank 2 (no codes). I figured if I started getting bank 2 codes, then the O2 sensor was to blame. If I continued to get bank 1 codes, then I have a bank 1 problem.

I disconnected the wiring harness so I could run a wrench to the sensor. I gave it the good ol' college try, but couldn't get the O2 sensor off. It was pretty late, so I just packed up the tools and went to bed. Drove the truck to work this morning (about 15 miles) and everything seemed to be running fine. Then I remembered once I got to work...."Hey, I forgot to hook the O2 sensor back up!" This caused a new code along with the p0171 (new code=p1131). I am going to clear the codes before I drive home from work tonight and see what happens once I get home.

Could the fact that, with the O2 sensor disconnected, it was running just as it was with the o2 sensor connected, suggest that I really need a new O2 sensor to clear the P0171 code?

Thanks!

 

What you should have (without checking) is a code for no signal from the sensor. When that happens it puts the engine into safe mode such runs rich. If that is running much better than before you have a problem for sure. I would replace that sensor, its not real expensive. You can get a loaner socket from the AutoZone,etc that fits over the wire. Soak it with PB blast and let the engine warm up. It should come out. Make sure to check your timing as well.