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The possible causes for a DTC P0421 (right side) and DTC P0431 (left side) are exhaust gas leakage, heated oxygen sensors faulty, a loose heated oxygen sensor(s), faulty Powertrain Control Module (PCM), or a WU-TWC deterioration or malfunction.
The most likely and common cause is simply a faulty oxygen sensor. First inspect the fuel cap to ensure it is tightened properly. If OK, inspect exhaust system for any signs of cracks or damage where gases can leak out. If OK, test the O2 sensors. If you need help with any of this so far just let me know.
2003 Jetta left rear window blue plastic part of the