How JustAnswer Works:
  • Ask an Expert
    Experts are full of valuable knowledge and are ready to help with any question. Credentials confirmed by a Fortune 500 verification firm.
  • Get a Professional Answer
    Via email, text message, or notification as you wait on our site.
    Ask follow up questions if you need to.
  • 100% Satisfaction Guarantee
    Rate the answer you receive.
Ask Eric Your Own Question
Eric, Automotive Repair Shop Manager
Category: VW
Satisfied Customers: 29729
Experience:  Automotive Repair Shop Manager, Technician
Type Your VW Question Here...
Eric is online now
A new question is answered every 9 seconds

04 Jetta T1.8 code PO420

Resolved Question:

04 Jetta T1.8 code PO420
Submitted: 6 years ago.
Category: VW
Expert:  Eric replied 6 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

Eric and 7 other VW Specialists are ready to help you

Related VW Questions