Use of Trouble Code Diagnostic Trees In Service Manuals

When using the diagnostic trouble areas in the Service Manual, release remember that the charts assume that you have a "hard" code. In other words, the problem has to exist right now. If you are dealing with an intermittent problem (a problem in memory), and the problem does not exist at the present time, you'll naturally not find anything wrong at any of the steps. Therefore, the charts will always tell you to replace the control unit. This most likely is NOT the problem. The reason the chart tells you to replace the control unit is because if you have a "hard" code and all the steps check out okay, what is the control unit complaining about? It must be no good.

Problems of an intermittent nature creating trouble codes are most likely poor or corroded connections or circuits that tend to go open or become shorted with vibration of the engine, or engine transmission movement, changes in temperature, moisture, etc. There is still a possibility that a computer or a component may be temperature sensitive and be intermittently bad, but a connection or circuit problem is more likely.