Had the codes printed out yesterday and where I'm sure one of the 02 sensors is bad, I don't know which one. Code is 172 H02S SYSTEM LEAN (BANK 1).
Which sensor?
There was another KOEO CODE 111 System pass code No faults detected during KOEO TEST

 

Is this problem on your 93 4.0L????

If so, I'd maybe suspect a vacuum leak, thats affecting bank one, maybe something like a intake manafold gasket leak, as the 4.0L is famous for that.

The computer is just saying that the O2 sensor is detecting a "lean" condition.

This doesn't mean te O2 sensor is bad, it's just reporting what it's detecting, thats coming from upstream of it, so don't shoot the messenger, at least not yet!!!!! lol

You might try rigging a "propane snffer" & pass it all around the bank one side of the intake manafold & see if the idle speed gets a increase, if so you've found the leak. Bank one is the passenger side of the engine.

If you find a intake side leak, you might try retorquing the intake manafold fastners, you might get lucky, if the gasket hasn't been sucked in & damaged, in which case it'll need replacing.

Let us know what you find.

 

Sorry . . . yes the '93 w/4.0L.

Soooooooooooo . . . what in the ??ll is a "propane snffer"?

 

A propane torch that ISN'T lit. Just turn the propane on and spray it along the sealing surface of the manifold and listen for the idle to change.

 

I like to rig my "sniffer" with a length of small dia. tubing & a "sports needle" (the kind used to inflate foot & basket *****), with it's end opened up, so there is just one hole, install it on one end of the tubing as the probe, the other end of the tubing attached to a propane bottle.

You don't have to turn it on very much to find a leak. If you can smell the propane, so can the engine!!!!

Any place where there is a vacuum leak, it'll "sniff" the propane & that'll cause the idle speed to increase. This won't hurt the O2 sensors or cat converter.

With the small dia of the needle, you can be very precicse in application & pinpoint some really small leaks.

With the vintage of this engine, also look for other leaks, like dryrotted vacuum lines, maybe the PCV lines too, but I think if they were leaking, you'd likely have a code from the Bank-2 O2 sensor, saying it too is unhappy, as a geneal vacuum leak like a PCV, would affect both banks.
So with just a lean code for Bank-1, I'd first look to that sides intake manafold gaskets, as I said the 4.0L has a history of intake manafold leaks.

Let us know what you find.

 

Could possibly be a bad injector on that bank too. Is there a definite miss in the way it runs? If one cylinder is getting real lean, the o2 sensor may not have enough adjustment to compensate for it.