Mitsubishi Montero & Montero Sport This sport utility vehicle offers more size than the other Mitsubishi SUVs, but manages to keep a sporty look and comfortable feel, unlike many larger SUVs.


Old 03-17-2019, 01:52 PM
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Join Date: Mar 2019
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Default P0420

Looking for some expert info on the P0420 I am now getting

replaced spark plugs, wires, valve cover gaskets etc. all seemed fine after the work. Drove entire tank of gas mo check engine light. 2nd tank of gas 50 miles later getting the code.

Some back info, when I pulled the spark plugs the tube seal on #3 was pinched upwards so tube was full of oil. I cleaned it out best I could before completing all the work. I also decided to put chevron Techron complete fuel system cleaner is first tank of gas after the work and no CEL. I did a 2nd round of cleaner in the next tank and got the code. Could the cleaner cause the code to appear? Just seems odd that no code during first tank and now there’s a code.

anyone with info on this will be appreciated. I hope I do not need to replace the cat.
Old 03-18-2019, 10:26 AM
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Join Date: Oct 2004
Location: Washington State
Posts: 1,311

How many miles you got on your Montero? Most likely your catalytic converter is just wore out and none of the "magic" cleaner solutions will work on it. I'd say save your money. There are several threats on the forum dealing with this that have very detailed information on possible fix. Do a little sluicing and searching.
Old 03-18-2019, 11:08 AM
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Join Date: Aug 2018
Posts: 29

If the previous owner had cleared the code, it is possible that the code took a while to come up again.

Please read the material at

Note that one issue is oil contamination of the cat. If you have bad tail pipe smoke, you have to deal with that before you can know whether you have a bad cat. This could include overfilling the oil and using the wrong oil weight, so consider those as well. Be sure your EGR system is working correctly. Be sure your PCV valve is clean. All of these issues can pour oil down the throat of the engine. After checking all of them, be sure to let the Cat heat up really well once to burn off anything that can be burned off, and then reset your code and see if it comes back. All of these are simply poor maintenance, and worthy suspects. I also changed (most of) my valve stem seals (which was a bit of a pain), and cut out most of the smoking of my engine.
Old 03-21-2019, 10:49 PM
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Join Date: Mar 2019
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It had 160k when I bought it back in January 2019. Its now at 186k. After using up the 2nd full tank with the Chevron Techron Complete Fuel System cleaner, I filled up another tank, cleared the code and so far it the code has not come back. I am thinking somehow that 2nd bottle using it back to back may have caused the code.

I will still check the egr and pcv just to be sure after doing all that previous work I mentioned.

thank you for your input.
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