2003 Hyundai Santa Fe P0401 and P0421 Codes After Timing Belt Replacement at 250,000 Miles

I have a 2003 Hyundai Santa Fe with 250,000 miles and a 4-cylinder engine. Recently, I had the timing belt replaced at a dealership. After returning, I noticed increased engine noise compared to before. A quick inspection revealed that the tensioner was allowing the timing belt to slip against the casing. The tensioner was subsequently replaced. Everything seemed fine for about two days. However, this morning while driving out of my neighborhood, I experienced severe loss of power—especially when climbing hills. Even with the pedal fully pressed, the vehicle struggled to move and barely crept uphill. On flat roads, the performance improved somewhat. Upon returning home, I checked the trouble codes and found P0401 and P0421. Any insights into what might be causing these codes or related issues after timing belt work?