So I got this the other day🤬

These went out all at the same time...
​​​​​​The Brick runs good, starts without issue, idles good too. But mileage is way down. Maybe 185 when the fill me up light comes on.
The 02 Sensors were all replaced three or four months ago with good Bosch sensors so I'd like to think that's not the issue. The only aftermarket in this is the 7 hole injectors which I believe may be causing it to run a bit rich. There's some soot on the tail pipe.

 

Had the front one on my '96 go out while on a trip. Mileage dropped 6-7 mpg.

 

No such thing, but I suppose that's debatable.

At any rate, I would check the ground circuit as they all share Z1 BK (looking at the 2000 SM).

 

The O2 sensor heater circuit gets it's power from a fused circuit in the PDC. The manual won't list it as a an O2 fuse but I'm pretty sure it's this one. The fact all 4 "went out" at the same time makes me think it's a fuse not the sensors themselves.

I got this picture from another thread on here but I think it's right.

 

I agree with downs.
When I first bought my Jeep pretty sure I had a P0135 too.
Traced it to the O2 heater fuse and pretty sure it was one of those 15 amp fuses in there that I replaced.
I waited for it to blow again, cause figured the fuse blew for a reason, but it never did. LOL.

 

I believe the Ca oxygen heater sensor system works on 2 relays instead of that fuse. Indeed, it doesn't even exist on 2000+ (on mine anyway):

 

My '00, CA Emissions, does not have a fuse there either but I believe they changed the PDC layout in 2000.
Pretty sure mine was a O2 heater problem, pretty sure it was a P0135, pretty sure I replaced one of those 15 amp fuses, and all was good.
It was literally one of the first things I fixed on the Jeep when I bought her, kinda running and not really driveable, 7 years ago.

 

Can't hurt to check 'em all!!

 

It looks like this guy feeds the relays. Check it out:

 

It'll run, but only in open loop which is safely rich. Therefore: poor mileage and sooty tailpipe.
You're on the right track!

 

Check for a burnt or frayed wire near the exhaust, usually Bank 2, sensor 2. Ask me how I know.....

 

There has to be a fuse somewhere that feeds those two relays.

 

That 20 amp mini fuse that Dave51 points out is absolutely the first thing to check. I have seen this multiple times; it's never been all sensors failing at exactly the same time. That would be one heck of a coincidence.

It is very likely one common resource causing this and often, it's that mini fuse.

 

Did some research and found this:


Mine is a 2001. I haven't looked but wondering if the number 10 fuse in the fuse box might also be the culprit. I checked the small 20 fuse in the PDC and it looks intact.

 

Fuses sometimes crack instead of blowing, so they still look okay but won't conduct electricity. Not common, but it does happen. Always verify with a meter or test light.