So, this story is not 100% DIY, but part of fixing things yourself is knowing your limitations and when to seek expert advice.
So, my 18 yer old car had a check engine light come on. I used my $40 OBD2 reader to find out what was wrong. It told me P0420. Crap! that's usually a bad catalytic converter. California compliant cats are very expensive and replacing mine would probably exceed the value of my old car. There is a slight chance it could be something less expensive like an O2 sensor, so I took it to my mechanic who has a much more sophisticated tool for analyzing what is going on in real time. He told me it was indeed the cat, but that the numbers where marginal. If we erased the code and ran it through some drive cycles, It might still be able to pass a smog test. He also recommended using a fuel additive to clean the fuel system and cat. He did not have much faith in additives, but it was worth a few bucks to maybe improve my chances. So, we cleared the code, I added some cat cleaner to the tank and the drove what seemed like endless drive cycles before the OBD said the cat was ready. I drove it to the smog check and it passed! I now can drive it for at least 2 more years until the next smog test is due.
