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Hello. I don’t know if I’m posting this in the right thread but I have a 2017 Elantra SE and the check engine light has been going on and off for months. I had it diagnosed at Auto Zone and it showed a P0301 code and said it was most likely the spark plugs and ignition coils. Had those changed today and the check engine light went off but then after I turned my car off and back on again it came back on. I’m also noticing the gas mileage wasn’t as good right after. I usually get 35-38mpg and on my way home it only got up to about 22…
What should I do from here?
 

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Step one would be check the error code(s) again to see why the light came back on.
If your original engine light allowed a lot of unburned gas through the system you could have killed the catalytic convertor.
 

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Hello. I don’t know if I’m posting this in the right thread but I have a 2017 Elantra SE and the check engine light has been going on and off for months. I had it diagnosed at Auto Zone and it showed a P0301 code and said it was most likely the spark plugs and ignition coils. Had those changed today and the check engine light went off but then after I turned my car off and back on again it came back on. I’m also noticing the gas mileage wasn’t as good right after. I usually get 35-38mpg and on my way home it only got up to about 22…
What should I do from here?
Code P0301 on the Elantra is a misfire on cylinder number one. Could be a coil, coil connector, spark plug, fuel injector, fuel injector connector, bad hydraulic valve lifter, bad valve, or bad head-gasket failing at cylinder number one. Typically for the Elantra, it is a bad ignition coil. I would have swapped two coils to see if the misfire followed the coil. Then that would have confirmed it was the coil and nothing more. Replacing parts at random is wasteful and often doesn't fix the problem. Proper diagnosis is the key.

Did you replace the plugs with the same brand and plug? Did you gap the plugs correctly? Did you purchase the correct high quality coils? Were the codes cleared after changing the plugs and coils? If you had other codes, it could be a number of other issues that would contribute to bad mileage. A bad fuel pressure regulator would cause a possible random misfire and bad fuel economy. There is a lot of unknown that is needed to properly diagnose.
 

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Hello. I don’t know if I’m posting this in the right thread but I have a 2017 Elantra SE and the check engine light has been going on and off for months. I had it diagnosed at Auto Zone and it showed a P0301 code and said it was most likely the spark plugs and ignition coils. Had those changed today and the check engine light went off but then after I turned my car off and back on again it came back on. I’m also noticing the gas mileage wasn’t as good right after. I usually get 35-38mpg and on my way home it only got up to about 22…
What should I do from here?
My question would be if you used HGK or Hyundai original plugs. I ask because for some stupid reason, auto part stores sometimes will recommend the wrong plug
 
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