On the 2.7L V6 and 2.4L 4 Cylinder, a defective exhaust gas recirculation (EGR) switching valve may cause an intermittent rough or unstable idle engine.
Problems for specific Hyundai Sonata years:
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Most reported 2009 Hyundai Sonata problems
Prior to replacing the automatic transmission fluid (to improve shift quality), make sure that the transmission fluid level is correct. Also, make sure the computer that controls the transmission has the most up-to-date adaptive programming information. When servicing these transmissions be sure to use fluid approved by Hyundai.
If you experience excessive engine noise on startup (after the engine has been turned off for several hours) that lasts for more than five or six seconds, turn off the engine and check that the engine oil level is correct. If the oil level is correct, the noise may be caused by the timing chain tensioner. Hyundai has an improved tensioner to correct this problem.
Problems with the accelerator pedal position switch may cause the Check Engine Light to illuminate; an improved switch is available to fix this problem.
Had my car serviced and one week later I had no electric, not even dash light. Put key in ignition it would not start. Got jumper power to battery no electric power and removed jumper. tried moving shift lever and suddenly had power and it started. Took it to dealer and they said could nog find as long as it was working OK. Week later it happened again. Jiggled the the shift lever again and it started again. took it to dealer and he said next ...