Most automobile owners have experienced their check engine light coming on at one time or another. The check engine light is also known as the malfunction indicator light (MIL), or service engine soon (SES) light. It is important to understand the reason why vehicles have a check engine light, what the light could potentially mean, and why it illuminates.

All automakers must comply with the Environmental Protection Agency (EPA) emissions standards if they want to sell new cars or SUVs in the United States. Every new vehicle manufactured in the United States or sold in the United States has to pass an EPA test which defines the acceptable limits of performance for the emissions control system on the vehicle. These limits define the conditions required for the check engine light to illuminate. If these conditions are met, the check engine light will illuminate. The reason a vehicle is equipped with a check engine light is to warn the driver there is something in the emissions control system that has fallen below the pre-determined minimum limit of performance in one or more of the systems being monitored. The check engine light coming on or being off is a result of on-board diagnostic (OBD) monitors doing their job and monitoring the systems related to emissions control.

The engine may run poorly when a check engine light comes on, but it also may have no impact on how the engine runs. When the check engine light comes on, there will be a data trouble code (DTC) or codes stored in the computer indicating the area or system where the problem is happening. At this point, the problem should be diagnosed by an experienced automotive technician.

The purpose of OBD readiness monitors is to self-test the vehicle’s emission systems and provide evidence these tests have been performed. There are continuous and non-continuous readiness monitors performing self-check routines while monitoring the performance of specific vehicle emissions control systems. Continuous monitors run all of the time as long as the key is turned on and/or the engine is running. Non-continuous monitors require certain conditions such as speed, acceleration/deceleration, fuel level, ambient and other conditions to be met in order for the monitor to run its testing sequence. Vehicles may perform up to 11 different system tests, and these are called readiness monitors.

The output of readiness monitors identifies whether the vehicle’s computer has completed the required tests while the car is being driven. If all of the vehicle’s equipped readiness monitors are ‘ready’ or ‘complete’, the check engine light bulb is working properly, the check engine light is not on, and there is communication at the OBD connection port, the vehicle will pass state mandated emissions or smog tests. This also indicates the vehicle is running at an acceptable level with regards to emissions and pollutants being sent into the atmosphere.

There are several reasons why the state of OBD readiness monitors are important if you are buying a used car. The vehicle’s computer can have its memory erased either purposefully or accidentally. This will temporarily turn off a check engine light and make all of the readiness monitors ‘not ready’ or ‘incomplete’. Some system monitors may take days or even weeks to perform their self-tests and turn the check engine light back on. A readiness monitor that is ‘not ready’ or ‘incomplete’ warns a potential buyer of several possible issues including problems with the system in question, an erased computer memory, recent repairs that may have caused the memory to be erased, or the vehicle’s battery may have recently gone dead.

Anytime you are considering buying a used car it is important to have the computer scanned to check the readiness monitors to make sure they are ‘ready’ or ‘complete’. Don’t be deceived. Make sure the vehicle you are considering has the readiness monitors ‘ready’ or ‘complete’. This will protect you from having potentially expensive surprises just a few days or weeks after you buy your vehicle.