Same here but I changed my catalytic converter and it went away a few days later it came back and went away again and came back and stayed on for weeks until I decided to put e85 in my car I have a 2017 focus sel and it's made to run it but I never ran it when I got the car till the other day but now my check engine light is off and the p0420 code is gone and still hasn't came back

 

This thread has not been touched in over 12 years. As for running e85, I am not sure any of the Montes were ever built to run e85 from the factory (if so, they would be 06 and 07 years). They can be made to run e85 with the right modifications.

 

the gas cap says e85 and the little door to but I've never put it in till now I've read through the manual about it but they recommend going back to 87 after the next oil change but all i know my car runs 10x better than it did before and it only dropped 3 miles per gallons so now a tank of gas last me 4 days usually it's 6 days on 87 I pretty much drive the same route every day

 

What year is your Monte? If it is a 2005 or older, e85 is NOT an option and can cause you some potential problems (this also means someone swapped the cap). If it is the 06-07 years, I believe GM offered use of e85 if you have flex fuel as an option.