2001 VW Jetta GLS 2.0L P0420 OBD2 Code: Causes and Fixes After Battery and Alternator Replacement

Model: VW Jetta 2001 Fault Code: P0420 Posted: 2008-05-29 17:33

My 2001 VW Jetta GLS with a 2.0L engine wouldn't start initially, so I replaced the battery. It started working for one day before dying while driving. I then replaced the alternator, after which the 'Check Engine' light turned off. However, three days later, the check engine light has returned and now displays OBD2 code P0420. I've seen several forum posts suggesting that a battery replacement can clear or resolve the P0420 code. Could a damaged battery caused by a failing alternator be responsible for triggering this code? Is it possible that the alternator issue is indirectly causing the P0420 fault? Any insights on diagnosing and fixing P0420 in a 2001 VW Jetta would be greatly appreciated.