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john, The Car Guy
Category: Toyota
Satisfied Customers: 16289
Experience:  ase tech over 15 years in the field
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Sienna: I am getting P0420 and P0430 codes and check engine

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I am getting P0420 and P0430 codes and check engine light on my 2004 Toyota Sienna. Started about when I hit 150k miles. I always get both codes at the same time. It is hard for me to believe both catalytic converters went bad simultaneously. Do you think I would have luck replacing both downstream O2 sensors?
Hi . Thank you for asking your question on JustAnswer.
I would start with the 02 sensors. Generally it's the fix. You can perform a back pressure test on the converters. But usually 02 sensors solve it... I would strongly suggest factory Toyota sensors

Thank you again for trusting us with your problem. John aka eauto
Customer: replied 4 years ago.

Thanks, XXXXX XXXXX alot of the research I have done.


I assume these would be the two downstream O2 sensors. Are these hard to replace?

Pretty easy they just unplug and unthread. You can use a 7/8 wrech. To unthread them
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