Thanks. I should have guessed that. No, CEL was never on for any reason. Dealer told me that mechanic who diagnosed and repaired the “Idle air control valve” did not notice anything wrong about EGR valve during his diagnosis. However, the frontman who test drove the car after repair noticed that car was idling rough. Then, they checked and found that EGR valve gets stuck time to time (I had to believe that because sometime I felt little rough idling.). And, they suggested that it will cause trouble in future. I thought it was due to “Idle air control valve”. The dealer told me that they did disconnect EGR to make sure that CEL deos not go on. Honestly, I do not know how they did it! Thanks again.