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Failed Smog Test

1997 Mitsubishi Montero Failed Smog For High HC and CO

My 1997 Mitsubishi Montero Sport v6, failed the smog test with high HC and CO numbers. I was told the EVAP system had an air leak. So what needs to be repaired? The tester didn't know.

I went to a Chevron station and the smog tech did an OBDII scan and told me that I had a air fuel leak, trouble code P0170. He couldn't get to really checking it because he was super busy. He couldn't really help give me any ideas on what to look for. I have been told numerous times "it could be this"....type answers, such as the fuel pump, fuel filter, fuel pressure regulator, massive airflow sensor, manifold pressure sensor and a faulty powertrain control module. I cant afford expensive mechanics. I usually diy it or have a friend/relative that does the hard stuff. HELP! What is the problem and how do I fix it ? Thanks, I hope you can help me.


This one might not be a DIY project, unfortunately. Without a range of testing equipment, it'll be very difficult finding the root cause of a P0170 trouble code. P0170 indicates the ECM (engine control module) detected a fuel trim outside the range of specification set by your vehicle’s manufacturer. The problem is, almost every single emission sensor on your Mitsubishi Montero is used to control fuel trim. Finding the responsible sensor(s), if in fact it's a bad sensor that's causing the problem, will require the inspection of system(s) responsible for the trouble code and high HC and CO readings.

The components you've mentioned, such as fuel pump, fuel filter, fuel pressure regulator, mass air flow sensor, manifold absolute pressure sensor, and powertrain control module, all can effect your vehicle's fuel trim.

We would have liked to be of more assistance, but unfortunately, when it comes to a broad trouble code such as Fuel Trim, the faults which may cause it are too many and can only be ascertained after an accurate hands-on diagnosis by an experienced smog technician.

posted by SmogTips Support 05-02-2017 11:26 AM

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