car losing pep uphills

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I have a 2003 saturn ion 2.2 auto tran. RECENTLY CAR HAS LOST ITS PEP WHEN GOING UP HILL OR INCLINE. runs strong down hill but slows to crawl going up. Feels like pulling a parachute. Changed plugs did not help. Not in firing,engine runs fine no cutting out or missing while this is going on. Took by autozone it read code p0420 which is a cross reference code with other brands i was told. Looking for place to start troubleshooting Tomorrow morning (monday0 Help