I have a 2006 Hyundai Sonata. Recently, the car can not go more than 55 MPH. I have changed the fuel pump gear oil, but problem still same. Can anyone help.

Have a 2001 Sonata. Mine did the same thing. I removed the O2 sensor from the catalytic converter. Sounded bad, but it ran great. Once I replaced the cat converter, & O2 sensors the car has run better than ever.

Change the fuel filter as well if you haven't. Check air filter. Check MAF sensor. Check engine light on? Take to auto parts store and have them connect an OBD scanner to detect problem, Usually no charges to test it. Start with the basics first.

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