I did my plenum gasket repair with a homemade AL plate a few weeks ago, and was wondering about the O2 sensor. The inside of the plate was covered in carbon and some oil, so it may have been leaking for a long time. My service engine light is not on and it seams to run a lot better since I did the repair. Is there any benefit to changing the O2 sensor or just leave it alone, since it seams to run fine.

 

Hows your gas mileage? Might wanna pull the pre-cat O2 sensor, and have a look, if it is covered in black soot, (burned oil...) replace it with a quality unit.

 

+1. Leave the rear sensor alone. All it does is monitor the catalytic converter, so no need to replace that unless it pops a code.

 

I will check it tomorrow. As far as gas millage, it sucks, but all of our trucks suck. I bought it with a blown plenum gasket so I really don't know what it should get. It has 203,000 miles on it.

 

if you do replace it, keep your old one for awhile.
some people report same or increased mpg. one or two reported decreased mpg and wish they had kept their old sensor to reinstall.

 

If it's got that many miles on it and you don't know the history, I'd replace the sensors. They aren't that expensive. I think yours has two pre-cat O2 sensors. Maybe $50 ea. I think there's a way you can check them with a multimeter if you want to check them before replacing them.
But on the other hand, that $100 may be better spent on:
a tune up (plugs, wires, cap, rotor)
Sea Foam the intake
air filter
oil and filter change and a can of Restore in the new oil
coolant change
change the axle oil
change the tranny fluid and filter and transfer case fluid
I'd also change the power steer fluid and break fluid (those are always neglected)
That's more than $100 bucks but stuff that probably needs to be done.

 

I tried getting the O2 sensor on the cat out, but it is in there tight. That will be another day. Any tricks to get it out, That is the only one I seen.

As far as the other stuff, I just changed the oil, filter and sea foamed it and replace the coolant, cap and rotor when I changed the Plenum plate. Wires look new, Air filter looks new, Transmission fluid looks very clean and red. Changed the brake fluid when I replaced the rear line rusted through near the tank. I have to replace the passenger side seal in the front axle, so I will replace the fluid then. I have not checked the rear. Power steering could use changing. I have not checked the transfer case fluid yet eather.

 

on the 94, you only have 1 O2 sensor, before the cat.

spray the threads with a penetrating oil like pb-blaster or other, a couple of times per day for a few days. the more the better.

 

Either that, or heat the little feller up with a torch..... Mr. Hot Wrench is your friend. Just stay away from the wires with it.

 

Another option would be to get the engine up to operateing temp. then try to remove it.
Just be careful not to burn yourself.
Then if need be let it cool down to put new one in.

Dave