Trouble code P0420 won't pass smog

Have an 04 Dodge Stratus 4cyl. Ck engine light on, manually reset 2 times comes back on after only 25 miles of driving but had almost no gas first time. Failed inspection due to ck. Engine light on. Got P0420 code. Light is off now (manually reset 2nd time) question is, if I fill tank with premium gas and/or use CRC0563 additive do you think I may be able to keep light off long enough to complete a drive cycle and pass inspection Don't want to go through the cost to fix the car as I will be selling it in about 2 months Just need a quick fix for now to pass inspection and be able to drive around for a few weeks without getting a ticket. Thanks! Oh should mention, changed oil, air filter about 600 miles ago, changed entire exhaust system inc. Cat. Converter about a year ago(i forget why but not for this issue) Car runs fine no other issues. I bought car about 18 months ago, had a good sticker so didn't worry about it, but check engine light has been on since day 1 with this car and the only code I get is this stupid P0420