2006 Chevy Cobalt P0420
Hello everyone I am looking for some assistance with my mom 2006 Chevy Cobalt with a 2.2 L 134 CID L4 DOHC 16V ECOTEC and 157 k miles. It is kicking a p0420 code and sometimes it kicks a p0300. Here is what happened yesterday. She drives like a Grandma so I told her she need to get out on the interstate and run 60-80 for a couple of miles and see if she could blow out the converter so It would pass emissions. She got up to about 75 for a mile or 2 and the check engine light started flashing. I checked the codes and it was a po300. I cleared the codes and took back off and it started getting a p0420. I cleared again and got a p0420 again. I did this 3 more time and each time p0420. I am assuming the converter is shot since it has 157k miles but my biggest question would a bad converter cause a p0300? I am going to get the car this weekend and change the converter and give it a tune up but I want to know what other stuff I should check. I have a code scanner and a Genesis Scanner but never used it. I also put in a can of seafoam in the tank this weekend.
Here is my plan.
Hook up scanner and monitor o2 sensors before changing the Converter and if they are showing ok change the converter.
Change Fuel Filter
Check Fuel Pressure
Change Air Filter
Check for Vacuum Leaks
Check Coil? How do I test it?
Can you think of anything else I am missing or anything else I can do? I have to get this thing to pass emissions by Feb 26th.
Any help will be greatly appreciated.