With the service engine soon light on, the car will alway idle high, as soon as the light is cleared it will go back down. It is the same as when you start the car cold, it will idle high and take a little to go down. I believe the reason is that when the car is started, the cats are not warmed up and the fuel/emissions system runs on an open loop system and does not use feedback from the O2 sensors to determine how much fuel should be added for complete combustion. This typically results in a running rich condition, as well as the decrease in gas mileage. When the check engine light isnt on, and the cats are up to operating temp the system will go to the closed loop operation and adjust fuel based on the feedback from O2 sensor. The check engine light forces the car to continue running in the open loop situation so this will have the effect of a high idle and poor gas mileage
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