2002 Montero Limited Oxygen Sensor Problem
 

Hello. I recently purchased a 2002 Montero Limited. Has a 178,000. Previous owner told me one of the oxygen sensors were bad. As I was checking the fluids, I noticed the MAF was unplugged, plugged it back in and seemed to run fine. After driving it for 30-45 min it started to shudder in 5th gear really bad. Pulled over, restarted and it didn't shudder. Drove another 10 min or so and it started shuddering again. After I made it home, I unplugged the MAF and it ran much better and the shuddering disappeared. I cleaned the MAF and plugged it back in and so far it seems to run ok with no shudder, but it is throwing 5 o2 sensor codes:

p0155
p0134
p0135
p0141
P0132

With the MAF plugged in, gas mileage is horrible and the exhaust smells pretty bad. Could a bad MAF trigger these codes? Or could something else be causing this? Thanks!

Yest is it possible. MAF is there so that ECU can meter out proper amount of fuel compared to how much air is coming into the intake. With the MAF unplugged (or not working properly), ECU running in a backup mode without any sensor data. That is good enough if you need to get home, but not something you should do driving every day. All the codes you listed are O2 related. But that doesn't mean it is O2 sensor issue. It can be, and two codes point to an issue with the heated circuit in the O2 sensor. Having a bad O2 sensor will also throw the ECU function into backup mode. Having both MAF and O2 bad will just complicate things further. The problem is that from your description it seems that there is no single condition at which the engine runs better or worse all the time. That means the issue is intermittent. It is possible that you have a bad sensor, or it can be a bad wiring or bad ground. You can just throw parts at it (replace sensors, replace MAF), but if this is a wiring issue, there is no guarantee that it will be corrected. Make sure your mechanic tests everything thoroughly before replacing anything.

Thanks for the reply! I going to check the ohm out the o2 sensors to see if they are good or bad. With so many codes being thrown and assuming after I check the o2 sensors and they are good, could it be possible the ECM is bad?

Wouldn't jump to that conclusion. Check voltage on O2s and make sure the wiring is good. Also, since MAF could be an issue - check that too. Test voltage and make sure you get change when engine runs at different RPM

I checked the all o2 4 sensors. 3 sensors did not ohm out and they correspond to the codes being thrown. Checked the voltage at all 4 plugs and they are receiving 12v. Haven't checked the MAF yet but will tomorrow.