Check engine light on on 2005.5 Jetta 2.5L

My mom purchased at 2005.5 VW Jetta 2.5L back in July. It has nearly 10K miles on it and it's been good until yesterday. The check engine light came on steady, no flashing. I can't tell any difference in drivability and everything seems normal. I'm hoping that it will only be a stuck thermostat or broken seal on the gas cap...something simple. Any ideas on what it is? I put gas in it the last time and I made sure the cap clicked 5 times. Also, I don't think it's an oxygen sensor because when my sister's Beetle had an oxygen sensor go out, the engine sounded much different and the car was VERY slow at low RPM.

Anyway, I'm taking it in tomorrow, but I wanted to see if anyone else had similar issues with such low mileage...

Jeremy

Okay, well I took the car in. They scanned it and found the auxilary air pump was to blame. There was an 'incorrect directional flow' reported. They cleared the light and told me that it must have just been a hiccup. It's been a week and light has not come back on.

Jeremy