Diagnosing an engine performance problem is almost a thing of the past without the aid of a Diagnostic Readout Box. On newer cars almost everything is controlled or affected by a computer. For performance the computer looks at several sensor readings and adjusts for fuel mixture and spark timing to keep the engine running smoothly. There are many variables to consider when diagnosing a performance problem but here are a few things you can do to help narrow them down.

- First of all, is the "Check Engine" light on or flashing? If the check engine light is on then you most likely have a sensor related problem. If the light is flashing then you have a major emissions problem and need to get it to the garage quickly. When the check engine light comes on that means the parameters in a sensor have gone out of range. It doesn't necessarily mean the sensor is bad because something mechanical may be causing it to go out of range to turn the light on as well. If you have a diagnostic readout box then you can at least see which sensor is out of range and of course read the code that is displayed in the computer's memory.

- If you don't have a readout box (which most people wouldn't) then there is another way to help determine the area of the problem. Having a repair manual for your particular vehicle will be a major help, make sure it's an electronic engine control manual with diagnostic information in it. You can often retrieve diagnostic codes from the computer without the readout box. On some vehicles (Chrysler is this way) you can turn the key on and off 3 times and with newer vehicles it will actually display the diagnostic code in the odometer display. Older vehicles will just give you a flash code and you will need to know how to read this, thus the handy repair manual.

- Once you know the diagnostic code and if the manual is extensive enough, you will need some electronic testing equipment such as an ohmmeter and a 12-volt tester. You can now make an educated decision on whether you have a bad sensor that is causing your performance problem. The repair manual should give you a step-by-step testing procedure to check certain sensors using your test equipment. Obviously you will need to have some common knowledge on how to use and read your test equipment.

- If the check engine light isn't on and you have a performance problem then you need to lean more towards a component failure such as spark plugs, coil, distributor, spark plug wires

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