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Greg, import technician
Category: Hyundai
Satisfied Customers: 7129
Experience:  27 year and 16 year as shop owner, import technician
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Hyundai Sonata GL 2003 sonata has a check engine light on,

Resolved Question:

2003 sonata has a check engine light on, when you turn it off wont start until it cools down, and has stalled three times in a week but starts back up once it cools down. Had a p0421 code.
Submitted: 6 years ago.
Category: Hyundai
Expert:  Greg replied 6 years ago.
Hi welcome to will answer your question the best I can,

Please do a good test drive for 10 mins then check for codes again.

Regards Greg
Customer: replied 6 years ago.
Same code
Expert:  Greg replied 6 years ago.
Hi .

A code P0421 may mean that one or more of the following has happened:

  • The catalytic converter is no longer functioning properly
  • An oxygen sensor is not reading (functioning) properly
  • A spark plug is fouled up.
But this code will not give you a no start problem after warm up?

Check the catalytic converter this could be blocking up and giving you this problem.

Best way to check this is when she stalls again is check and see if you are getting spark to your spark plugs.

If not then you may have the typical cam/crank sensor fault ?

When theses sensors malfunction they will not always trip a code?

Need further assistance please let me know.

Kind regards Greg

Greg and other Hyundai Specialists are ready to help you
Customer: replied 6 years ago.
How do I check to see if there is a spark to my plugs?
Expert:  Greg replied 6 years ago.
You will have to pull out a coil from the cylinder head and connect a known good spark plug, then ground the spark plug on the top of the head valve cover and crank the engine over and see if the plug is sparking?