The secret of effective diagnosis and troubleshooting is to have a logical, well-ordered system. Following a logical step-by-step procedure will get you to the root cause of a problem quickly and efficiently. Repair manuals will have a diagnostic tree that goes in a logical progression. It will ask a question, usually yes or no, and depending on the answer will branch off in two or more directions. The last box in the tree will have the problem and how to correct it. The example shown is a simple one. A more complicated system will have branches going into specific tests and then have 2 or more branches from there depending on test results.You will need some basic test equipment to perform your diagnosis. A 12-volt test will tell you if a circuit has power or, when hooked up in reverse, will tell you if a ground circuit is good. A good volt-ohm-meter (VOM) will be needed to perform specific voltage and resistance tests. You can get a good meter at any Radio Shack or Best Buy store. A digital meter is an excellent choice because they are easier to read than an analog meter. I have both digital and analog meters in my toolbox because sometimes an analog meter is best for a certain job. Most VOM’s have an ammeter that will test the alternator output and test for current draws. Make sure the one you buy has it.If you have an older car with a point equipped ignition system, you will need a dwell meter as well to measure and adjust the dwell angle of the points. As with any piece of equipment, read the instructions that come with your meter. It will tell you what the various functions are and how to connect the meter to the circuit for accurate test results. Most meters have a fuse in them to protect them from an incorrect connection. Make sure the one you buy has one and get a couple of spare fuses. You will, at some point, hook it up wrong and be very thankful you have the spares. I know I have.Most of the troubleshooting on today’s cars will be electrical in nature. A good wiring diagram is essential to properly troubleshoot any electrical circuit. They usually come in two parts, a schematic and the wiring diagram. The schematic shows the different components of a system and how they relate to each other. The wiring diagram shows the actual wire colors and connections.For testing the mechanical side of the engine, you will need some more specialized equipment. A vacuum gauge with several adapters can be used to test manifold vacuum and test vacuum operated circuits. A hand vacuum pump is handy for testing vacuum operated components such as EGR valves and heating and air conditioning systems.Once that is done, you need to check the basics. I don’t know how many people get burned looking for a complicated answer when it’s a simple answer. If your house started leaning to the left, you wouldn’t check the roof first. You would check the foundation. Same thing with a car, the foundation has to be sound before you look anywhere else.With a drivability problem, the first thing you should check is the spark plugs. They will tell you a lot about the condition of the engine if you know how to read them. Check to make sure they are the correct plugs for that car. Don’t automatically get the same ones that were in there. Check them. Maybe the guy who tuned it up before you put the wrong ones in. Use AC Delco plugs in GM, Champion in Chrysler, Motorcraft in Fords and NGK’s in Japanese cars.After the spark plugs you need to check the ignition wires, distributor cap and rotor.Sponsored LinksMake sure the plug wires are tight on the plugs and there are no cracks or burns on them. A quick test is while the car is running, spray water from a spray bottle on them. If the car starts to stumble or run rough, or you see sparks arcing, you need new wires. Look inside the distributor cap for cracks and burns. If you see any, replace it. The same goes for the rotor. If the tip is burned out, replace it.If in doubt, replace any of these parts. The cost is small and you will know that they are in good shape to continue troubleshooting. These are the most common causes of misfires and rough engine performance. The next thing to check is the vacuum lines. Make sure they are connected and in good shape. Trace the whole line for cracks, breaks and collapsed areas.Murphy’s Law stipulates that a cracked or broken vacuum line will be in the most hidden place in the car. After that you need to check for loose electrical connections. Unplug the connectors and look at the terminals. Dirty, loose or corroded connections will cause a world of strange symptoms and intermittent problems.Check the filters as well. A new air and fuel filter will solve quite a few drivability problems.Do the same thing for electrical problems. Check the fuses and fusible links first. Check connections to see that they are clean and tight. Check light bulbs to see if they are just burnt out and the correct type. I had a customer bring a car in and said every time he stepped on the brake his dash lights came on. What he did was put the wrong type of bulb in his brake light and it would feed back into the dash. A dishonest mechanic would have had a field day with this one. He would replace the bulb and charge 5 hours labor locating and repairing a short.In short, check all the stupid things first. Don’t take anything for granted. Car won’t start? Check the gas gauge first. That one burned me a couple of times.When you look for an electrical problem look at it from all angles. Literally look at it forward and backwards. Will it be easier to trace from the component to the fuse box or vice versa? If you are tracing a wire, is the connector buried in the dash? Look at the wiring diagram and find another location to make the same test in a more accessible location. You’ll get the same results in a much shorter time.Lastly check the computer for Diagnostic Trouble Codes (DTC’s). For this you will need a scanner or a service manual that explains how to pull the DTC’s up manually. If you get a bad injector circuit code, don’t assume it’s a bad injector. That code is telling you it’s in the injector circuit and that includes the wiring, injectors, dropping resistors (on older EFI cars) and computer. Any one of which will throw that code. Get your wiring diagram out and check the whole circuit. Too many parts get replaced because “the computer said it was the fuel pump.” Don’t get burned buying an expensive part only to have it not fix your problem.When you have found the problem and completed making the repair, check it again. Make sure it is fixed. Sometimes one fault will cause another one that you couldn’t see at first. For example, there is a shorted wire between the injector and the computer. You fix the wire and take it for a ride and it still misses. Well you didn’t see that the shorted wire burned out the injector. When you test the circuit again, you will see that the injector needs to be replaced also. Double-check your diagnosis; double-check your work and double-check the repair.Troubleshooting a problem can be tough, even in the best of circumstances, but by following some simple rules, using the right test equipment properly and some common sense, you will locate and repair most problems with your vehicle yourself.