One problem related to gas recirculation valve (egr valve) has been reported for the 2017 Hyundai Elantra. The most recently reported issues are listed below. Please also check out the statistics and reliability analysis of the 2017 Hyundai Elantra based on all problems reported for the 2017 Elantra.
Below 50 degrees it takes the heater an extremely long period for the car to reach normal operating temp and then when the heater is turned on it falls back to almost into the cold position and begins to blow cool air. The temp gauge will then go up while driving but will go down toward the cold position while idling. My other car is a 2011 Elantra and have no issues with it as it warms up in just a couple of miles and stays warm and blows hot air. Dealership has checked it out and says its working fine but there are several cases on the internet reporting the same with other owners and even found where hyundia has issued a TSB replacing some thermostats in the 2017 models.