One problem related to gas recirculation valve (egr valve) has been reported for the 1999 Hyundai Sonata. The most recently reported issues are listed below.
The engine light continues to go on whenever I change gas stations, or level of octane when I put gas in my car. This will also cause the car not to start occassionally and emit black smoke from the exhaust before starting again. I brought it to the original dealer (milford, CT) numerous times where they just reset it because they could not locate the problem. I have since moved and brought it to the new local dealer (greensboro, nc) who also reset it and suggested I replace two exhaust system parts because they "think" that's what the problem is (possible leak in the exhaust system, however the mechanic clamped off the hoses at different points and could not determine there was actually a leak). These two parts combined cost over $500. 00. That's a high price to pay for something they "think" would solve the problem.