Hey y’all, my names Jake and I am the owner of a 2003 Chevy Blazer LS. Bought the thang about 7 months ago with a temporary inspection sticker. Now I’m in need of an inspection, but I’m pulling up both the P0420 & P0300 faults. I’ve done some work on it myself already to not only remedy the issues for inspection, but also as a much needed tune up. I’ve completed the following repairs/maintenance the past week, completed the drive cycle, and still pulling both the P0420 & P0300 faults:
- Replaced radiator
- Replaced radiator fan blade
- Replaced thermostat w/ o-ring
- Performed coolant flush & added new coolant (DEX)
- Performed air filter, oil, & oil filter change
- Replaced fuel filter
- Replaced spark plugs & wires
- Replace distributor rotor & cap

Like I said after completing this list, I’ve completed the drive cycle & pulled both the P0420 & P0300 faults. I’ve used the Actron OBD2 tester to check the faults & obtained a read out of the ECM data. The first two upstream O2 sensors seem to be fluctuating normally, but down stream one is fluctuating between .16 & .90 volts, which I know should fluctuate slightly around +/- .50 volts. I’m not having and ignition or idling issues, the catalytic converter seems to be rattling, and it doesn’t FEEL like I’m misfiring at all. My question is: Could the main issue be a faulty catalytic converter causing both the P0420 catalyst & P0300 random misfire codes? Or does anyone have any other usual information or advice on what I could do next to get these faults to clear? I really need this thing to pass inspection, like in October 2020. Thanks y’all!