For the longest time I kept getting P0431 code on my full size 2001 Montero. I found new cats online, had them installed this week by local muffler shop.
The day after, check engine light comes on. I read the code today, and instead of P0431, like before, it is now P0155.
Research shows it might be the sensor or wiring. Has anyone else had this issue? Truck seems to be running fine - it is irritating after spending 900 dollars it throws another error.
I live in CA, so I have two more years to figure it out before the next round of smog detecting ***** come knocking...

 

That code is for the heated (upstream) O2 sensor on driver side. Start by unplugging and reconnecting the sensor, check wiring on the harness and on the sensor side. It is possible that this is just a bad connection, loose/broken wire or the sensor is bad. This should not be too hard to trace. Not related to your cats, with the exception that the sensor had to be removed (and possibly damaged) when removing old/installing new cats. You can have the same shop do the checking, and if they find the sensor or wiring is damaged, you can argue that it was them who caused the issue (i.e. warranty on the work)

 

I bought the Y pipe online and had it replaced. Foolishly, I never looked at the pipe close enough to see how many sensors are in it. I thought there were only two. I had sprayed a break-free compound on the two sensors I knew about to help out the muffler shop when the time came to replace them. All I did was get down on the driveway, look under the Montero and aim and spray. I didn't see any other sensor.
You mentioned they had to remove the heated O2 upstream sensor when replacing the cats. I am assuming that sensor is also in the new Y pipe. Where can I look to see the sensor? Is it hard to get to? I'm by no means a mechanic, but not an idiot either.


If the wiring was damaged, it would make sense to me that the computer would throw the code immediately. I reset the code and drove it on the highway for two hours and it did not come on. Then the next short drive I took, the light came on. That said, I'm assuming the sensor is damaged/bad. Just thinking out loud - I do wonder if the previous code masked the new code, or is it possible that the new code is a product of the replacing of the cats? There is no doubt in my mind that the new code is related to the changing of the cats.

 

The upstream (or so called "heated") O2 sensors are installed right before the cat converters on the Y-pipe. The driver side one is actually on the Y-pipe and passenger side (Bank 1) is on a little elbow extension.
Passenger side is very easy to get to. It is sitting on a horizontal part of extension and is facing backwards. Driver side it a pain in the rear - it is up in the tunnel where the manifold is going down to the Y-pipe. Very tight space and not much room to swing a wrench.
It is possible that the sensor or its wiring got damaged during work performed on the cats and Y-pipe.
You need to test continuity in the heated circuit of the sensor and to see if it is getting proper voltage. If you do not have tools for this - take it to a mechanic. They should be able to test it and determine if it is wiring or the sensor itself. When I say "damaged wiring" it can be as simple as a pin in the connector not making proper contact with the mating end. That is what you need to test/check

 

I found the sensor right where you said it was. Kind of a pain in the rear to get to, so I didn't. I found the electrical connector under the hood, quite easily accessible. I disconnected it and cleaned the connector with contact cleaner and put it back together. That didn't fix it. Reset the computer, and code came back quickly.
I don't know what to check on the connector. I'm a electronics tech by trade, so I have all the tools for electronics work. Just don't know what to look for on each pin.


I checked online pricing for the sensor. I thought I read that it was 30 dollars somewhere, so I checked and it came up as this:


$253.25 list. Part number is MD369190.


Do you suppose I am being quoted the right sensor? Holy cow!!! that seems a little pricey.

 

The price you list for OEM Mitsubishi sensor sounds about right. Unfortunately are heavily overpriced. I've been running AC Delco in mine for several years now and they are working just fine. If you order an aftermarket O2 sensor, make sure to get one with the vehicle specific connector and not the one with just a pigtail.
Attached is a PDF with the diagnostics for P0155 for a 2003 Montero. The steps for your diagnostics should be very similar. I hope this will help.

 

I used Denso O2 sensors that I purchased from Rock Auto as part of my rebuild of my 2002 3.5L 6G74 in my Montero Sport
Post 119 of this thread https://mitsubishiforum.com/forum/mi...-48920/page12/