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My Honda Civic's check engine light (MIL) keeps alerting me to P0300 Random Misfire codes. I have tried multiple times to repair it but the light keeps coming on.

Understanding how your car's computer tracks and calculates misfires will not directly help you to fix the problem, but perhaps some general knowledge will help you take your diagnosis to the next level.

Your engine compresses mixed air and fuel, which is ignited, and combustion occurs, producing power to turn the engine's crankshaft. Worn spark plugs, faulty injectors, internal engine failure and air leaks are just a few trouble spots that can cause the fuel to not burn completely. When any amount of fuel is left unburnt, the crankshaft slows down ever so slightly and the corresponding decrease in power is monitored by the car's computer.

Many manufacturers use a similar system known in techie parlance as the science of angular velocity measurement. When the crankshaft slows, the computer interprets it as a misfire and its internal misfire counter starts keeping track. Once the computer arrives at an unacceptable number of misfires, it turns the MIL on.

Random misfire codes are the computer's way of telling you that it sees a repeating crankshaft slowdown, but can't determine its cause. Your job is to figure that out. Tune-up items are the most obvious area, but if these aren't the problem, there is oh so much more to consider.

Editor's note: With this column, Lou Trottier, owner-operator of All About Imports in Mississauga, begins a weekly item about maintenance and repair. Have a question? E-mail globedrive@globeandmail.com, placing "Lou's Garage" in the subject area.

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