Why is my "check engine" light going on after being serviced?

Question : "I was preparing for a road trip to Florida to investigate where to send my parents now that I'm reasonably confident that I can make it on my own… so I take the car over for an oil change, general inspection, and tire rotation. No problems, everything is done while I wait and the Service Advisor tells me that everything looks good, it doesn't really need any additional work right now. So I stop at the gas station and fill the tank, drive through the carwash, and head home to pick up the family and get going. Not two miles into our trip, I notice the "Check Engine" light on the dash is glowing. What should I do?"

There are literally hundreds of conditions which could cause the check engine light to illuminate, but if the car is running normally, first thing to do is check to see if you left the gas cap loose when you filled up. Since the advent of OBDII (on-board diagnostics, second generation), the most common cause for the check engine light for many cars is a gas cap left loose by the driver who just filled his tank. You see, the evaporative emissions system runs a periodic self-test to check for any leaks, and as part of this test the computer turns on a pump which is supposed to pressurize the fuel tank. If you have left the gas cap loose, it will detect a leak and turn on the malfunction indicator lamp.

On some cars, after you have tightened up the loose gas cap and as you continue to drive, the cars computer may re-test that system and turn the light off if it passes. On some other cars, you will have to take it to the shop and have the light reset. If the light stays on for more than a couple of days, or if any other symptoms arise you should schedule it into the shop for a scan test and diagnostic. (If the light is flashing, you need to stop driving immediately and have the car towed to the shop). If you do take it in, be prepared to pay between $50 and $100 or more for this testing – it takes some serious training and some very expensive tools to properly diagnose these modern day rolling computers!