Hey Scotty, I have a 2018 Hyundai Elantra with 36,000 miles. I had a P0304 code, changed the spark plugs now it’s gone, but now when I come to a stop the rpms fluctuate with what feels like engine braking help.
could be a problem with the power steering/hydroboost. Should be under warranty right?
Check the spark plugs and spark plug wires. If they are good, you need an advanced scan tool to get the live data to diagnose. @mountainmanjoe mentioned a good point. Do you have warranty on that car?
Sorry for the late reply, yes for factory warranty. I took it to Hyundai twice, first time they updated the software, it was still doing it but not as bad and the second time they said it’s okay. They Drive tested it with the scan tool hooked up and said there’s no problems.