Okay, so your engine light is on and you want to know why and what to do about it. Read on:
So why is it on?
Your engine computer (ECM or ECU) is constantly receiving data from all the engine subsystems. When it repeatedly receives inappropriate data, it flags the event by storing diagnostic trouble codes (DTC). The codes indicate specifically (I use the term specifically very loosely ) what is wrong. The ECU will also light the check engine light on your dash.
So, can I still drive the car?
Maybe. If the engine is behaving strangely, stalling, shaking, bucking, smoking, knocking, rattling, or anything it doesn't do normally, then don't drive it. BTW, if any of the previous conditions are normal for your car, get them fixed. If the light is flashing (not just on) then don't drive it. It means that there is a misfire and unburned fuel can enter the catalytic converter and ignite. It's not as dangerous as it sounds, but it can ruin your expensive cats. Misfires are also stressful for your rods, crank, and engine mounts.
How do I get rid of it?
Slow down. It's really best to find out what's going on. Some auto parts stores like Autozone will read your codes for free. You can also find another member with a code reader or buy one of your own. If you own a VW, I'd recommend a Vag-Com from RossTech. Once you have the codes, google them and you should be able to find out what they mean and sometimes a solution. Adding the make and/or model of your car will help narrow the search.
Can I just clear the code and cross my fingers?
Sure, if there is a real problem it will come back to haunt you soon enough.
I don't have time to figure out what's going on. Can't I just do a bunch of random stuff in hopes of fixing it?
Maybe. There are eight subsystems that fall under OBD-II standards- EGR, O2 sensor heaters, O2 sensors, air conditioning (?), secondary air injection, evaporative emissions, catalyst heating, and catalytic converters. Just about anything could be causing the code(s). Here's some things you may want to check first. Make sure your gas cap is on and seated at least three clicks. Open the hood and take the fancy covers off the engine. Look around the engine bay for any disconnected connectors or cut wires. Also look for any frayed or cracked vacuum lines.
So how do I know if my repair worked?
Many of the DTCs will clear themselves after a certain amount of good data is received. This sometimes can take weeks. The best thing to do is start off fresh by resetting the ECU. The same person who read your codes can do this. Resist just disconnecting the battery. You may cause your fuel system to reset requiring a throttle adaptation. You will also put your radio in safe mode.
So I'm all clear. Can I get my emissions test now?
Sort of. The eight OBD-II categories have readiness codes. All eight need to be at ready to pass an emissions test. A fifteen mile drive should do it, BUT you need at least one start from cold and part of the trip has to be at highway speed. Turn on the AC once too.
How much will this cost me?
How much you got? Luckily, most states have a limit on what you need to spend before they give you a freebie. In many cases, the fixes are as simple as connecting a vacuum line or connector (about $150 at your local dealer).
For search purposes- OBD OBD-II DTC MIL CEL emissions emmisions emmissions check engine light malfunction indicator lamp diagnostic trouble code