Montero O2 sensor

You just made the single most common DIY mistake by replacing whatever part is listed in the trouble code. You didn't take the time to see that codes refer to a circuit malfunction. Check the fuse. If the fuse is good, get a wiring diagram and check for bad splices. You have a wiring problem, not bad sensors

P0141 O2 Sensor Heater Circuit Malfunction (Bank 1 Sensor 2)
P0161 O2 Sensor Heater Circuit Malfunction (Bank 2 Sensor 2)

Here's a link to check out. How to Diagnose P0135 Heater Performance Bank 1 Sensor 1 GM 3 5L

their are 2 O2 sensors one before the cat and one after ,try changing the other one first before you go to a new cat which is expensive to say the least

The oxygen sensors, or O2 sensors, on a Mitsubishi Montero Sport are located under the vehicle. Models made prior to 1996 had only one sensor which was located on the catalytic converter, behind the exhaust manifold. Newer models have additional sensors located on the left and right sides of the manifold.

P0130 - O2 Sensor Circuit Bank 1 Sensor 1
P0135 - O2 Sensor Heater Circuit Bank 1 Sensor 1

Here is a visual aide that will clear up the confusion on where the affected sensor is located.

The error code should tell you which bank and sensor you are looking for. with that information:

bank one on a montero sport is on the passengers side.
bank two is the drivers side.

sensor one is before the converter,
sensor two is after the converter.

each bank has two sensors. on each bank there is a sensor with a black and a gray connector, and no they will not interchange. connectors are different.

besides disconnect the actual o2 sensor, you can just disconnect the plug to be sure you got the right one. the plugs are hidden in the sides, just follow the cables. you could disconnect the plug of the one that you thing is the wrong one and check again the error code to see if you are getting the same error. if you are getting a different one, then you disconnected the wrong one.

good luck with the change. some wd40 or liquid wrench will help with the removal of the sensor.

replace the o2 sensor the one before the cat converter with a quailty sensor like a denso brand o2 sensor

YES. but the dealer should have been able to tell which one was bad. I would say they replaced the wrong one, having more than 1 bad is really rare.

The O2 sensor is mounted to the exhaust; you'll need to get under the truck (jack-stand please) to locate. If it sits where a 89-92 sits it will be in the exhaust under the passenger side. Follow the wiring, on most it carries over the transmission to a wiring block under the driver's sie of the truck (again this is all under the truck) As for the low idle I do not believe there is anything to be change. If the idle is low it is due to other conditions... injectors, sensor, low compression, timing or such.
I hope this may have helped,
Tom

MR988905 costs $300+

SNG O2 sensor is just below $100

they may claim being made with the same quality and both are tagged "direct fit" for easy installation.

its just my idea, good luck.