Technical Topics No. 15

CHECK ENGINE. There's no fun about those two words and there's also not a lot of logic to be gathered from them either. Check engine? Could they be a little more specific? Unfortunately, no, they can't and that's because the Check Engine light comes on if anything isn't 100% under the bonnet. This means that you could be staring at a major repair, or your fuel cap could be too loose (no kidding).

All modern vehicles have a computer or the ECM (Electronic Control Module) that controls the operation of the vehicle powertrain (the engine and transmission). The main purpose of this is to keep the engine running at top efficiency with the lowest possible emissions. With constantly growing demands for better fuel economy and new strictest emission regulations it's not very easy to achieve. The engine parameters need to be constantly and precisely adjusted according to various conditions such as speed, load, engine temperature, gasoline quality, ambient air temperature, road conditions, etc. That's why today's cars have much more electronics than in early days - there is a large number of various sensors and other electronic devices that help the vehicle computer or ECM to precisely control the engine and transmission operation and monitor emissions.

The vehicle computer system has self-testing capability. When the computer senses that there is a problem with some of the components it stores the correspondent trouble code(s) in its memory and lights up the "Check Engine" light to tell you that there is a problem and your car needs to be looked at. To properly diagnose what is wrong, you need to take your car to a mechanic or a dealer. The mechanic will then hook up the scanner to the car computer and retrieve the stored trouble code(s). Then he will look it up in the service manual provided by a car manufacturer. The service manual contains the list of possible codes (about several hundreds) and describes what each code means and what needs to be tested. The code itself doesn't tell exactly what component is defective - it only indicates where to look, what engine parameter is out of normal range. The mechanic will have to perform further testing to pinpoint a defective part.

Unfortunately, the majority of recurrent Check Engine episodes eventually lead to some professional repair time. The most common problems that trigger the light are emission control malfunctions. The emission control system is what your car uses to try to keep our air a little cleaner. To do this, it employs dozens of sensors, valves, flaps, heated wires and probably some fairy dust. Every car made in the last 20 years has at least one oxygen sensor, and they don't last forever.

But don't throw your wallet away just yet. There are also plenty of little things that can make the Check Engine light come on, and many are easily corrected. Here are a few of the more commonly occurring issues:

Your fuel cap isn't on tight enough.
You read that right, it might be your fuel cap. Some cars measure how much pressure is building up inside your fuel tank. It involves a series of mathematical algorithms that track your driving style and how much pressure is usually in the tank, then set off an alarm if it strays a certain percentage from the average. All is means is that if you're fuel cap isn't on tight, it thinks something is up and lights the dashboard Check Engine light. Tighten the fuel cap and see what happens. It may take a week or more before the light goes out.

Your engine got wet where it didn't like it
Any electrical hick-up under the bonnet can cause one of your car's many sensors to take a funny reading. When it does, you can expect to see the Check Engine light. We have seen a vehicle triggered the Check Engine light every time it rained. After a lot of diagnosis, we found water that was dripping onto a spark plug wire, then running down the wire to the engine's head, causing an occasional short. Every time the water ran down the wire, the light came on. A few days later, it would turn off on its own. Be sure your engine doesn't have a wetness problem. More common than rain water getting in there is the overzealous owner who sprays his engine down at the high pressure car wash, shooting water into every crevice of the engine, thus lighting the light.

Your spark plug wires are bad
As your spark plug wires start to get old, they may develop tiny cracks which can let little bursts of electricity out. This electricity was supposed to be going to a spark plug, and since it didn't, the engine will misfire slightly, meaning one of the spark plugs didn't spark enough. Once again, this can cause the Check Engine light to come on. With your engine off, check your spark plug wires for tiny cracks or holes, especially around the ends of the wires. If they look shabby, you should replace them.

At some point the powers that be decided that it would be easier to figure out what was wrong with your car if it just told them what the problem was. Sounds like a great idea, right? In some ways it was, but there are 10,000 different diagnoses your car can give, so putting your finger on the right one can be difficult.

Figuring out how to fix the problem is another story, but the first step is to know what your car is trying to tell you. The language it speaks is OBD, which stands for On Board Diagnostics. If something goes wrong, a mechanic (or you if you have a code reader) can plug into your car, and will get a numeric code telling them what is wrong.

Often your car will let you know ahead of time when something is wrong by turning on the Check Engine Light. Unfortunately there are dozens of stupid and unimportant reasons for the light to come on. But checking the code will give you something to go on whether it's important or not.

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