my engine management light wont go off ive taken it to a garage and they are saying nothing is wrong with the car but i have noticed a considerable difference in the acceleration of the car does anyone know anything that it could be???

I also have had a check engine light on my volvo for the past 6 weeks. I took it to a mechanic and they ran a computer diagnostic that idendified a problem with the thermostat. He said it was not overly serious and could wait a while before being fixed. I then called and told the volvo dealer about it and he said that even if the problem is not serious, if the light is on the car goes into "safe mode" which may affect performance.