Your Vehicle’s Check Engine Light

It’s not your father’s idiot light….

How much do you know about your “Check Engine” light? If you’re like most people, not much. Most owners only know that they don’t really want to see it come on while the engine is running. The “Check Engine” light came along when computerized electronics took control of powertrain management. For most manufacturers this was in as early as the 1980s. Of course the early computer systems, and the check engine light that accompanied them, had some pretty big imperfections. Consider them developmental bugs. In large part the issues were due to a lack of standards from make to make and model to model. Eventually, however, standards were developed and the systems improved.

Progression of the check engine light and electronic engine management systems can be divided into three stages. The initial use of these systems is usually referred to as “OBD1”. OBD stands for On-Board Diagnostic and the “1” means first generation. OBD2 was mandated in all vehicles after 1996 and was the first to use a standardized protocol which was mandated by the EPA. The third progression is being referred to as C.A.N. (Control Area Network) Capability. This protocol, also sometimes referred to as the early stage of OBD3, has the all of the functionality of OBD2, yet it requires the communication and interaction of the various computers within a vehicle. This means that for the first time an issue read by the computer controlling your transmission can be communicated to your engine’s computer, allowing the computer to make adjustments.

Since these requirements came about via the EPA it makes sense that when the check engine light comes on there is some issue causing your emissions to be more than one and a half times the federal standard. Many people figure “So what? My inspection isn’t due, why should I care?” In fact you should care very much because this light is a final warning that a failure will cause damage to critical components, which may lead to even more expensive repairs down the line.

Of course, newer cars have much more sophisticated controls and much more sensitive tolerances for emissions so it is possible a vehicle can have a drivability issue without the check engine light ever coming on. The newer systems are even capable of shutting down troubled cylinder injectors and ignition in order to keep emissions in check. Overall performance suffers, but no increase in emissions occurs. Another aspect of the EPA protocol is that whenever a problem exists, a DTC (diagnostic trouble code) must be stored in the memory of the ECM (electronic control module). This provides some insight as to what the issue might be, or at least where the issue is being detected.

Let’s say for example, code P0341 exists. This is a code for “oxygen sensor and related system.” There are several possibilities for this code to appear. A bad oxygen sensor? Possibly a bad injector spraying excessive fuel into the engine, thereby raising the emissions reading? Or another common culprit, a simple vacuum leak, which has been the issue on 75% of Fords and Mazdas in our experience. The code only provides the affected parameters and components. It does not isolate all possibilities.

It is still left to the mechanic evaluating the entire system and going through the process of elimination to pinpoint the offending cause. And no, we don’t have a magic box to tell us exactly what part needs to be changed! In fact, every manufacturer has their own strategy for which components perform what role on it management function. So next time your check engine light comes on, please understand: It is a warning about higher-than-normal emissions and to turn it off, the issue must be addressed beyond just taking out the check engine bulb. It is a very sophisticated system in which the final and last line of defense lights up to warn the driver that there is eminent trouble.

This article first appeared in the July 2005 Edition of The Exhaust, the monthly newsletter of the Z Club of Houston.

About Kin

Z World is owned and operated by Kin Chan, an active member of the Z Club of Houston. Kin is an ASA-certified and ASE-certified mechanic with over 20 years in the automotive service industry.