How Important is that Check Engine Light?
My Check Engine or Service Engine Soon light just came on, what do I do?
If the light is on steady you usually have time to schedule an appointment for repair, or even drop by a repair shop and have them "pull the code" so you will know what system it might be in before you schedule an appointment.
If the light is flashing, you need to get it in as soon as possible as more expensive repairs may be needed if the problem isn't attended very soon.
The on board computer controls spark timing, fuel injection, emission controls, transmission operation, cooling fans, cruise control, climate control, speedometer, anti-lock brakes, skid control, climate control and many other accessories. It receives inputs from many different sensors and input devices and sends control signals to valves, controllers and other output devices. When the computer sees a problem with one of these inputs or can't control fuel mixtures or something else properly it turns on light and sets a trouble code.
The Check Engine light can come on to warn you about problems that can cause poor mileage or performance such as a bad oxygen or throttle sensor. It can also be a warning that you may be left by the side of the road one of these days. Other warnings can be about things that can cause catalytic converter, transmission or engine problems. As the computer also monitors emission systems any problem that might increase emission output will also turn on the light.
Why do you need to fix the problem if the car still runs good?
If a sensor quits working or is supplying bad information the computer will often compensate by using another sensor to calculate the information. If that sensor fails also, you may be left waiting for a tow.
If you ignore the light because of a problem that doesn't seem to affect the way the car drives, something else might crop up that will cause big problems and you may not know until it's too late because the light is already on. Unfortunately you only have one check engine light.
Replacing the component that is listed for the code doesn't always fix the problem as something else may be at fault. An EGR code can be caused by a bad sensor or solenoid or even plugged passages. Oxygen Sensors are often replaced for an Oxygen Sensor code and the problem is actually caused by things such as a bad Air Flow sensor, fuel pump, vacuum leak or any other thing causing a fuel mixture problem.
We've seen bad alternator diodes cause codes for camshaft or other sensors.
Early systems (1980 - 1995) were called On Board Diagnostics I.
Systems since then are called On Board Diagnostics II and monitor a lot more systems.
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