Cadillac Repair: Check Gas Cap message, emission control system, gas station attendant

Question
I had Auto Zone read the service engine soon code and it is P0440 Evap emission control system malfunction. I had the code reset but the check gas cap message came back on again after about 150 miles. Thank you for your previous reply. Do you have any new ideas with addition information I have given? Thanks, Darlene
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The text above is a follow-up to ...

-----Question-----
Hi, I have a 1999 Seville SLS with 95K . The "check gas cap" message came on two days ago along with the "service engine soon" light. Three weeks ago it was just the "check gas cap" light on. I was low on fuel and after filling up the message did not come on again until two days ago when I had half tank of fuel. Do you have any ideas what might be wrong? Should I buy a new gas cap? I appreciate your help. Darlene
-----Answer-----
More than likely the gas station attendant or yourself did not tighten the gas cap tight enough after you put gas in it. The light should go away by itself in a few days.

Answer
1. make SURE that the gas cap is on properly.

2. If it is on properly the light should go out by itself in a few days of driving the car.

3. If it does NOT go off after a few days of driving the car you will need to have the code tested to see why it is coming on.

If your mechanically inclined, I can e-mail you the repair procedure. E-mail me at: onlineautohelp@msn.com