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Eric, Automotive Repair Shop Manager
Category: VW
Satisfied Customers: 31907
Experience:  Automotive Repair Shop Manager, Technician
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04 Jetta T1.8 code PO420

Resolved Question:

04 Jetta T1.8 code PO420
Submitted: 7 years ago.
Category: VW
Expert:  Eric replied 7 years ago.



DTC P0420 - catalyst efficiency below threshhold. This basically means that the readings received from the post catalytic convertor O2 sensor are below what they should be. Possible causes:

  • failed catalytic convertor
  • exhaust leak between the exhaust manifold and catalytic convertor
  • damaged wiring harness to downstream (post) O2 sensor

The vast majority of times it is due to faulty convertor.


Check for exhaust leaks and repair as necessary. If ok, check wiring harness for damage at the downstream O2 sensor.


If the above are ok, replace catalytic convertor.


The catalytic convertor is covered by the Federal Emissions warranty for 8yrs or 80,000 miles, so if you are under 80,000 miles it will be covered by this warranty

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