Also called the "Check Engine" light or "MIL", (malfunction indicator lamp). The engine computer receives voltage readings or signals from various sensors and it controls a number of functions and circuits. A defective sensor will report a voltage outside predetermined limits which will be detected by the computer.
The computer also performs system tests while you're driving. Most notably, it tests for leaks in the fuel tank, and it opens a valve to purge the charcoal canister of stored fuel vapors so they can be burned. The computer watches various sensors during these tests. It can detect a problem with the test results, and it can detect a problem with things in the circuit based on an amount of current flow different than expected.
Even if sensors are working properly, they can indicate problems with how the engine is running, for example the exhaust might be constantly too rich or too lean, both of which lead to increased emissions. The computer also tries to reconcile one sensor with another to check their accuracy. For example, if the engine has been off for more than eight hours, the coolant temperature sensor and the intake air temperature sensor had better be reading the same. Likewise, the throttle position sensor could have a very high reading indicating heavy acceleration, and the MAP sensor could have a very high voltage indicating coasting, but they better not show up at the same time. In other words, the computer can even detect a problem from sensors that are working correctly.
Starting with the '96 model vehicles, an extra oxygen sensor, (or two) was added to the exhaust system. By comparing the two signals, the computer can determine the efficiency of the catalytic converter. His job is to burn any unburned hydrocarbons. They are so effective now that the exhaust is almost entirely carbon dioxide and water vapor.
Any time a problem is detected, a diagnostic fault code (DTC) is stored in the memory of the engine computer. Many customers think these codes tell the mechanic which part to replace, but it's not that simple. The codes tell which circuit or system has the problem, not the specific part. They also tell the improper operating characteristic of the engine that has been detected, but not the cause.
When a problem has been detected that will adversely affect tail pipe emissions, the engine computer commands the Check Engine light on to let you know. The severity of the problem affects the operation of the light. A problem with the air conditioning clutch relay, which is run by the engine computer, will set a fault code but the Check Engine light will not turn on because that problem doesn't affect emissions. "Running cold too long" also doesn't turn the light on. Every Chrysler vehicle up here in Wisconsin has that code every winter because it takes longer than six minutes to reach full operating temperature unless you floor it on the highway within seconds of starting it and leaving your driveway! That code doesn't turn the light on either.
For a relatively minor problem, the light will turn on while you're driving, indicating something needs attention, and it will turn off if the problem goes away. If the problem is a little more severe, the light will turn on and stay on until you turn the engine off, even if the problem goes away, but the light will be off next time you start the engine. It won't come back on unless the problem acts up again.
In still more severe cases, the light will come on when the problem occurs, and it will always be on every time you start the engine, even if the problem never acts up again. The worst case is when the light is flashing. Bail out and dive for cover! Well, pull over and stop the engine as soon as possible and let the exhaust system cool down. Something is happening that is causing way too much unburned fuel to enter the catalytic converter. A fouled spark plug is a typical cause of too much unburned fuel in the exhaust. If you keep driving, the converter will start to glow red hot. The catalyst will melt and plug the exhaust system.
When a fault code was memorized on older cars, if the problem was corrected, the code was erased automatically after starting the engine about 50 times, or typically in two to three weeks. I don't know if that is still the case with a 2008 model. You used to be able to erase codes by disconnecting the battery but that defeats their purpose. The fault code descriptions provide extremely valuable information to the mechanic. Erasing those codes throws that information away and makes it harder to diagnose some problems which, in the long run, costs you more money.
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Sunday, January 10th, 2010 AT 4:04 AM