Technical 20 www.aftermarketonline.net OCTOBER 2026 Execute diagnostic plan: Testing with purpose, not guesswork In the latest article in our series from Darren Quinn of Mechanic Mindset, we explore the third stage in the ‘FIXDIT’ diagnostic framework: eXecute Diagnostic Plan. Other articles in the series can be found on the Aftermarket Magazine website at tinyurl.com/AM-FIXDIT as they are published. In the previous articles, we gathered the evidence and then inspected that evidence to build a logical theory. Now we reach the third stage: eXecute Diagnostic Plan. This is where thinking becomes testing. But the objective isn’t to test everything. It’s to perform the right tests, in the right places, and understand what the results mean. Every test should answer a question It’s easy to become busy during diagnostics. Connect the scan tool. Check some live data. Measure a few voltages. Scope a sensor. Swap a component. Before long, plenty of tests have been performed, but are we actually any closer to solving the problem? Before carrying out any test, ask: “What question am I trying to answer?” If you can’t answer that, consider whether the test needs to be performed at all. A good test should confirm or eliminate something from your theory. If you suspect a component isn’t operating because it has lost its power supply, for example, your test should be designed specifically to prove whether that supply is present and capable of carrying the required load. The measurement isn’t the objective. The answer it provides is. Know what good looks like Before taking a measurement, you should also understand what result you expect. A reading of 12.2V might initially appear acceptable on a nominal 12-volt circuit, but what is the circuit doing at the time? What voltage should actually
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