What to do if SES/MIL lite is on
 

Many members post questions about their car running poorly and not addressing the fact that the SES lite is on. Their first step would be to get the car scanned and see what the complaint is about. Here is note from the NY DMV regarding the light.

What is the warning light on my dashboard that displays the words, "Check Engine", or similar words? What can I do if the warning light illuminates?
The warning light is the malfunction indicator light (MIL) of the OBDII (or OBDI) system. The color of the MIL is red or yellow. The MIL can display the words, "check engine soon", "service engine soon", or words that are similar. The MIL on some vehicle displays the image of an engine with the word, "check".

The MIL normally illuminates when you insert the key and turn the vehicle ignition toward the position that starts the vehicle engine. If there is no problem, the MIL does not remain illuminated while the engine operates. If you start your engine and the MIL remains illuminated, the OBDII (or OBDI) system has found a problem.

The problem may reduce fuel efficiency, reduce performance, increase air pollution, or cause harm to the engine. Take your vehicle to a qualified automotive technician to determine the severity of the problem and if service or repairs are required. Normally the illumination of the MIL is not an indication of a malfunction that requires immediate attention, but an unresolved malfunction may lead to more serious problems or costly repairs.

To indicate a possible serious malfunction, the MIL flashes on and flashes off while the engine operates. Take your vehicle to an automotive technician immediately, and only operate your vehicle if necessary. Do not operate your vehicle at fast speeds, and do not use your vehicle to carry or tow a heavy load.