New montero sport questions
 

Hi guys. I just bought a great condition 01 Montero sport. So far its great fun to drive and for the most part runs good. I do have a Few questions. So CEL is on. I had it checked and it vcame up as o2 or cat 's . swems this is a common issue. The only trouble I seem to see is when the engine is at operating temp it idles funny. Will idle around 800 rpm and then drops to like 400 chugs and then back up. Never dies but almost. Would this be due to the bad cat/cats? Or is their somthing else I should check?

What is the code? The "O2" code is too vague to be of any use. Also if you have a freeze frame from the codes, post those too.

Here is the codes

Well, It seems that two of your codes are most likely related - Lack of O2 sensor activity can be causing the Lean condition. You'll need to check/replace Left Front O2 sensor (Driver side). The Lean condition may be causing the unsteady idle you describe. I wouldn't be panicking over it - its fixable. If you are lucky - the sensor gone bad and replacing it will take care of those issues. If you are not so lucky, you may have an issue with the wiring harness. But again, addressing it will take care of the O2 problem and the Lean condition on Bank 2.

The catalytic efficiency code is for a different bank (passenger side) and you will not fix it with the O2 sensor. You most likely have a worn out catalytic converter.
Do you plan to do the work yourself or have to have someone do the work? Let me know if you need help. You are only few hundred miles away from where I'm :)

Iv done o2 before but changing out the cat looked pretty complex. Their seems to be multiple cats as well as prices

Cat replacement should be fairly straight forward process as long as (just like you pointed out) you get the right parts. If it is a "bolt on" unit it should be easy. If you are getting a universal kit that you have to weld, that it a whole another story. Just make sure the parts dealer double check fit with your VIN number. Also, get the hardware - bolts, nuts and all gaskets. You'll may need to cut off few fasteners and new ones will save you lots of headache.

So I plan to do the cat replacement somtime in the next few months. Is it worth it right now to replace the o2 sensor now or just wait until I do the cat? Would it help the idle issue?

I think you should replace the O2 that is giving you the P0154 code right away. That would be driver side front sensor. The reason you want to replace it right away is this sensor is responsible for giving the engine computer information needed to optimize the fuel mixture. If it's not working (no activity), the computer has to run in a "safe" fall back mode that is not optimal for the conditions. You can get new O2 sensor, swap it and hope it works or you can do some testing first. Swap the left and right front sensors and see if the error "jumps" to the other bank. If it does you know 100% that you need a sensor. If the error stays on the same bank, you have a connection/harness issue. Replacing the sensor in the second scenario will not fix the issue, just waste your money.
Good luck