I just picked up my S40 a few weeks back and have only 650 miles on it. This morning my check engine light came on. My dealer told me that this can happen if the gas cap isn't screwed on until it clicks. The car manual backs this up. What causes this? If this is the problem, will the check engine light shut off after I tighten the cap and re-fill it? Thanks for the help, James.

 

If the car completes 2 or 3 driving cycles and doesn't see the fault again the light will go off.The code will still be in the computer system but the light will be off.

The reason the light will come on if the cap is left off is because.The cars does a self test on the fuel system and if there is a leak in the system it will tell the computer.If the cap is left loose it creates a leak.If it does the test 3 times and doesn't see the leak it will turn the light off but still store the code.