Hello,

I have an engine code said 0420 P0420 Catalyst System Efficiency Below Threshold. What does this mean? please explain and let me know what I should do. Thank you so much

 

WHAT YEAR ???

The code means is the secondary O2 sensor (behind the cat) is reading a higher than normal amount of unburned oxides.
If you live in a state with emission testing you might need to replace the converter.
If you do replace the converter save the old one for the the recycler. Some of the older Cats are worth $50-70.

You might consider giving your accord information in the signature of your post.

 

Thank you HONDADUDE,

My Honda is 2003 Accord EX, V6.