Well.... Try as I might, I am totally stumped on this one. Hopefully one of you gents can save me from further countless hours of confused tinkering on my Disco.

So here is the current puzzle.
I have a 97 Disco 1, Auto Trans, 146k miles.
I keep getting a CEL with two codes. P1138 and P1158, both I believe are 02 sensor rich codes. I also have some fuel trim weirdness (or at least I think its weird...) both bank 1 & 2 have LTFT of -5.47% and STFT of 24.22%.

I replaced both upstream o2 sensors with oem replacements. I also removed and inspected the two rear o2 sensors, cleaned them up a bit, and reinstalled.
On torque. I'm showing all 4 o2 sensors reading at a constant 1.125v regardless of engine temp.
My idle is high, but varies between startups. It is usually sitting around 1600 rpm
I also cleaned my MAF, it is reading at 0.9 g/s at idle.
Im currently working on getting emissions checked but I'm showing my o2 heater as "incomplete", cats incomplete, and evap sys incomplete.

Over the last few months I've also done the following (if any of it seems relevant)
-new TPS, it now gives a reasonable reading.
-new (salvage yard) IACV
-cleaned throttle body
-fixed various vac leaks and now have a vac of 29.8 in/Hg
-changed valve cover gasket
-new alternator
-new battery
-fluid changes. all of them.
-new serpentine belt

Any ideas on where to go from here? The constant voltage on the 02's is the most confusing part for me.

 

Those codes are for both back sensors I had a similar problem, swap both of your back sensors to the front and both front sensors to the back, then clear the code drive it till it throws the next code, you should get a code for the front sensor or sensors, then replace the bad sensor. the problem I had was that when the back O2 sensor fails the computer will throw a code or codes but it will not tell you which one of the two is bad so by swapping the rear to the front and the front to the rear it will then give you a specific code as to which one has failed. Give that a try.

 

Thanks for the ideas! Ill give it a shot!

I thought that the rears were just sniffers and the fronts gave input to help determine fuel trim. Am I confused about that? Either way, It sounds like your suggestion is worth a shot!