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See also this article here, Why would you pay to diagnose a check engine light?

Your car has a computer that controls the amount of fuel added to the engine to maintain an optimal air fuel mixture, adjusts the the ignition timing for the best performance, and monitors emission control devices to make sure they are working properly. If the computer knows of a problem, it needs to alert you, so you bring the car in for service. This is the role of the check engine light.

The check engine light looks different from car to car, but it's always a yellow / orange color on the cars we repair. If the check engine light comes on solid (not blinking), you can continue to drive the car until you have a chance to bring it in. This does not mean you should ignore it, regardless of what your brother's friend who knows about cars says.

When the check engine light is on, the car is likely to be wasting fuel, polluting excessively, causing premature part failure, or perhaps all three. Driving around for a short time until you can set up an appointment is no problem, but continuing to drive for months or years is a bad idea. If the check engine light is blinking rhythmically, there is a problem the computer thinks may cause damage to your car. If you notice a blinking check engine light, you do not need to stop in the middle of the bridge and call a tow truck (like you would with an oil light), but you do need to stop driving as soon as practical. Whether the check engine light is solid or blinking, it's a good idea to drive gently.

Your engine control computer monitors many sensors to help it make decisions. It looks at engine RPM, throttle position, coolant temperature, ambient temperature, air intake, vehicle speed, and oxygen content in the exhaust, to name just a few. When one of these sensors tells the computer something the computer knows can't be true or is not optimal, the computer will turn on the check engine light and set a "code". When you bring your car in to the mechanic (hopefully Art's Automotive), the technician will connect a scanner to data link on your car and "pull codes". The code will correspond to a sensor or sensor circuit. The mechanic uses this information to get a better idea where to start looking for the problem.

 

However. This is step 1 in the process, not nearly at all what constitutes diagnosis. Find out why pulling codes n' pluggin' parts is NOT what you're paying for in this article.

 

For instance, if there were a "code 1" on an older Honda, the mechanic would know to check for problems with the O2 sensor, wiring, or other problems that might affect output of the O2 sensor, like a misfire. On 1996 and newer cars the computer stores a lot more information, much like the "black box" flight recorder found on airplanes. When accessing the computer on newer cars, the mechanic will get not only the code, but also freeze frame data at the time the code was set. Information like how fast the car was going, whether the car was warmed up or not, how hard the gas pedal was pressed, etc. This information can be quite useful when trying to understand what happened or getting the problem to recur during testing.

As useful as all of this information is, it's only a very small part of the diagnostic process. Once the code is known, the real testing begins. Many people place too much emphasis on the magic power of the computer to diagnose the problem. The scanner can not tell a mechanic what is wrong with a car; it just gives a direction for further investigation. If we were to simply replace whatever sensor matched the code we pulled, our success rate would be well under 50%. With some sensors costing more than $1000, this would make for some very unhappy customers.

Let's say you have an older Honda with a "code 1", indicating a problem with the O2 sensor. If you just replaced the O2 sensor after pulling this code, you might not fix the real problem. Code 1 is usually caused by a misfire or lean mixture, not by a bad O2 sensor. The O2 sensor and computer only know what the oxygen content of the exhaust is, not why it is what it is. When O2 sensors fail, they are unable to read rich conditions and tell the computer the mixture is too lean all the time. (Lean is indicated by a lot of oxygen in the exhaust.) When the computer sees the O2 sensor reading lean all the time, the check engine light comes on and a code 1 is set. One might think, "oh, I'll just replace the O2 sensor". But what if the exhaust has a high O2 content not because it's lean, but because there's a misfire?

When the engine misfires, it pumps the unburnt air fuel mixture (containing lots of oxygen) into the exhaust. The O2 sensor is tells the computer it's lean all the time and an O2 sensor code is set. The computer is just incapable of any real analysis of what's happening. It's up to the mechanic to figure out what's really wrong.

Last Updated on Wednesday, 05 January 2011 10:20  
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