This topic contains 4 replies, has 2 voices, and was last updated by 
Richard Kirshy 2 years, 11 months ago.
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Topic
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Installed new spark plugs a few weeks ago and now getting P0420 codes… is it just a coincidence or could the new plugs not be working correctly?
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Sorry, failed to state it’s on a 2014 Ford Focus with 137k miles.
Are they the MFG recommended plugs?
Are they gaped correctly?Yes correctly gaped. I chose better then oem plugs from rockauto.
If the plugs are correct and properly gaped.
Then your left with the following possible causes:\
Faulty oxygen sensor
Faulty air-fuel sensor
Worn or internally failing catalytic converter (Most Common)
A leak in the exhaust system
Misfire (the root cause of converter failure)
Rich or Lean air-fuel ratio (the root cause of converter failure)
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