I’ve stumped the local dealership and their Hyundai Tech Line as well. This is a 2011 Hyundai Sonata with about 50k miles. Also this is my first winter with it so I can’t tell you if it’s a new issue or if it’s always happened in the “cold”. This is Northern California so the cold nights are about as low as 30.
On a cold start, after 20-30 minutes the heater sputters out and the lights on the control unit go out. Sometimes it goes back on if I continue driving for 5-15 more minutes; another way for it to go on again is to turn the car off and start it again. The final way to turn it back on (which was discovered the last time it was in the shop) is to gently begin to start the car again (while it’s on) and just that slight movement in the ignition cylinder turns it back on.
The sputtering out doesn’t actually turn it off completely, just the lights, buttons and knobs stop responding and the heat mostly goes off.
The control unit has been replaced, plugs have been tightened and the ignition cylinder has been replaced.
They’ve road tested it 3 times and I’m a little tired of having to get a rental car as they claim to not be able to start a new case with Hyundai Tech Line until they have my car.