We have all been there.
Driving along when that dreaded light on your dashboard lights up and says "Check Engine".
So what do you do? Immediately pull over to the side of the road and call the tow truck? or just continue on thinking that you'll take care of it later? I know I've been guilty of the latter.
What should you do? In talking with my mechanic buddy I learned that when that light comes on you need to observe what the light is doing to decide what your next step should be.

1. Occasional flashes indicate a momentary malfunction.
   
2. If the light comes on and stays on it means the problem is more serious like an issue with your emissions output or even a vehicle safety issue.
   
3. A constantly flashing 'Check Engine' light means its time to pull over and think about that tow truck because there is a major problem.
   

Whenever the Check Engine light comes on the vehicle's computer will record all the sensor readings at the time of the malfunction.
This is known as a 'freeze frame'.
This picture of the status of your engine is very valuable for figuring out what the problem is.
This information is accessible with the use of an auto code reader either by your mechanic or by you if you have a code reader.
The reader hooks up to a plug-in easily found under your dash and will give you a code that corresponds to the malfunction.
Depending on the scanner it will tell you what part is responsible, how to repair it and also reset the Check Engine light.
The OBD ii code scanner that your mechanic has is more advanced than what the typical vehicle owner has access to plus he has the experience to determine what caused the particular part or sensor to malfunction.
Owning your own OBDII scan toolmakes sense if you want to know what the problem is before you go into the shop.
It can save you the cost of the diagnostic scan and if you are handy working on cars it can really save you some cash.
Just use common sense before you start replacing parts to fix the problem.