Only 3 things that can cause a misfire? I don’t think so.

It has always concerned me when in classes on computer control, the instructor would say. “Remember techs, you must first diagnose any computer control vehicle as if it did not have a computer. You must “check the basics” first.

Even a well respected national training company steers the techs with a slide related to what can cause a misfire by stating that there are only 3 things that can cause a misfire – Ignition, air/fuel, and mechanical. I think they missed something. There are 4 things – the one they missed is “significant resistance in a shared current path”.

We all know that a bad alternator can produce enough energy to operate the ignition system, and just about nothing else.  Shared current path resistance can do the following:    Lower voltage, which lowers amperage to:   Fuel pump, Ignition primary , PCM that provides ground for the Fuel Injectors.

Voltage pushes current. Shared current path testing is smart troubleshooting.

My book “Understanding and Troubleshooting Vehicle Voltage Drop” has a section beginning on page 37 and ending on page 49 that covers shared current paths and shared current path testing.

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