Check engine lights can seem mysterious. Understanding how a check engine light occurs greatly increases the likelihood of repairing the issue. Generally speaking, it’s all just voltage.


Battery voltage enters the engine control unit. Battery voltage will be about 12 volts, but may be higher or lower. In order for the computer to work accurately, it needs a steady and constant voltage supply. The computer therefore takes battery voltage and reduces it to reference voltage. Reference voltage will be much lower than battery voltage, and commonly is 5 volts

The reference voltage then enters a sensor of some kind. Based on what the sensor is seeing, it will have a varying level of resistance. A temperature sensor may increase in resistance as the temperature increases. The amount of resistance combined with the known reference voltage allows the car to calculate the sensor’s environment (temperature, throttle position, etc)

If 5 volts enter a sensor that has 2 volts of resistance, then the computer will see 3 volts as the signal voltage. The car then compares what the sensor is reading to what it expects to see. If the voltage is not within the accepted levels, a check engine light results. The process by which a car sends reference voltage to a sensor and the sensor returns a signal voltage for the computer to read is known as closed loop diagnostics. The voltage is sent out, and returned, completing a loop.

So far, it’s fairly simple. The difficulty occurs when there are multiple components working together in a loop.

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Whatever your relationship with the automobile, it is important to understand who built it. The trouble with auto manufacturers is that they are always building stuff for each other. This post will help sort out who built what, for who, and why it matters.

First thing first, let’s get acquainted with the major automotive families. Automotive families are different makes that are all made by the same manufacture. Many times manufactures will want to produce a regular run of vehicles, and then a more luxurious run of models. They will produce what is essentially the same vehicle, but badge them differently and pack one with all the high end wonders people love.

The major automotive families include …

Toyota, Lexus, and Scion. Honda and Acura. Nissan and Infinity. Hyundai and Kia. BMW and Mini. Volkswagen, Audi, Skoda and Seat. Jaguar and Land Rover. Ford, Lincoln, and Mercury. Dodge, Chrysler, Jeep, Plymouth and Eagle (Here shortly, toss in Fiat). General Motors, Chevrolet, Oldsmobile, Cadillac, Pontiac, Buick, Hummer, Saturn, and GMC (As well as Opel, Vauxhall, Holden and Daewoo). You can always tell a terrible product, because they will have numerous names for the same bad product.
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The other day a Nissan came into the shop. The battery and the alternator had been replaced. The car would start if you drove it, and let it sit a few hours, but would need a jump start if left over night. The battery tested good, and the alternator was charging plenty of amps, with no A/C ripple, and 15 volts. Sounds like a classic case of a draw.

I hooked up a test light and found the draw is on the fuse for the alternator charging signal. (For information on how to find a draw please visits here ) Occasionally, you will have an alternator that is charging fine, but back-feeds when the car is off and thus results in a draw. Given the fact that the alternator has already been replaced, it is highly unlikely that the alternator is the cause of the draw. So where do we go from here?

Upon inspection of the alternator, I found the following
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The other day I was installing an axle. When I pulled the new axle out of the box, an instruction sheet was present. I opened it up, flipped through it, and found an interesting noise/ vibration chart for drive line problems. Complaints, or problems, are across the top row, and possible causes are listed down the left hand column.

noise complaint chart
(click to see full size image)
This chart is interesting, accurate and a helpful place to start diagnosing drive line noise or vibration complaints. None of the information is earth shattering, but it is handy to have in a chart. More importantly, the lesson learned here is that regardless of how many times you have made a repair, if there are instructions with your replacement part, take the time to read though them, you never know what you might come across.

When it comes to automotive repair, there are several methods of approaching the problem. Let’s look at: known problems first, maintenance as pair, comprehensive versus fix just what is broken, as well as a few diagnostic approaches.

Known Problems First

Until we correct all the known problems in a system, we can not know if they are effecting the portion of the system we are concerned about. Often times tracking down the cause of one problem will reveal the cause of others. Use this method with a bit of common sense, replacing a gas cap for an evaporative leak will most likely not fix a misfire. However, often times vacuum leaks cause many, many problems. (For a specific example of fix know problems first, visit here )
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Every repair order should include the four C’s. The four C’s of automotive repair are, complaint (or concern), cause, correction, confirm. As a technician, utilizing the four c system will ensure you repair a vehicle properly. Similarly, as a customer, looking to see that the four C’s are on your invoice will help ensure you are using a mechanic who knows the proper steps to repairing a vehicle.

We will use the example of a car overheating to demonstrate how to use the four c method. The repair order would say something in the realm of: “Customer states vehicle runs hot, please advise”. As a technician, your first step is to replicate the complaint. Whenever possible, quantify the complaint. Test drive the vehicle and watch the coolant temperature sensor. You can record the data, and add to the repair order, “Test drove vehicle, engine coolant temperature reaches 230 degrees Fahrenheit”.
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