Execute diagnostic plan: Testing with purpose, not guesswork

1 October 2026  

In the third 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 by following this link , with more to be published in the coming months.

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 be present? Is the circuit under load?

The same applies to oscilloscope waveforms, pressure readings, current measurements and scan data. Without knowing what good looks like, a measurement is simply a number.

This is why technical information, wiring diagrams and known-good data are so important. They allow us to establish a pass or fail condition before the test is performed.

Ideally, every test plan should answer three questions:

1. What am I testing?
2. What result do I expect?
3. What will I do next depending on the result?

That final question is particularly important. A test should determine your next move, rather than simply adding another measurement to the job card.

Divide and conquer

An effective diagnostic plan should also eliminate as much of the system as possible with each test.

One of the simplest techniques is the half-split method.

Imagine a component isn’t operating and there is a long electrical path between the power source and the component. Rather than checking every connector and section of wiring in sequence, choose a convenient test point somewhere near the middle.

If the supply is good there, you’ve potentially eliminated everything upstream. If it isn’t, your investigation moves in the opposite direction.

Repeat the process and the area containing the fault becomes progressively smaller.

The same principle can be applied beyond electrical circuits. When diagnosing a noise, you might first establish whether it comes from the front or rear, then left or right. On a network fault, a strategically chosen test point may help isolate one section of the network from another.

Good diagnostics is often as much about proving what isn’t faulty as finding what is.

Choose the test to suit the fault

Different faults require different testing techniques.

Consider the three common electrical fault types: open circuits, short circuits and resistive faults.

A continuity test might help locate an open circuit, but it can be misleading when looking for unwanted resistance. A circuit may show continuity with a multimeter yet fail when asked to carry current. In that situation, a voltage-drop test under load could provide far more useful evidence.

This is why there isn’t one universal electrical test.

The same principle applies to diagnostic equipment. Oscilloscopes, multimeters, current clamps, pressure transducers, thermal cameras, smoke machines and scan tools all provide different information.

The skill isn’t owning every tool.

It’s understanding which tool and test will answer the question most effectively.

Don’t try to prove yourself right

There is another important discipline at this stage.

Remember the theory developed during Inspect Evidence? Don’t become attached to it.

The purpose of your diagnostic plan is to prove or disprove that theory. A result that proves your theory wrong isn’t a failed test – it is valuable evidence that prevents you from continuing down the wrong path.

When a test contradicts your expectations, stop and reconsider the evidence.

Sometimes the quickest route to the fault is proving your first idea was wrong.

Diagnostic insight

Before connecting any diagnostic tool, ask yourself: “What result would make me go in one direction, and what result would make me go in another?”

If the answer is “I’m not sure”, the test probably isn’t ready to be performed.

FIXDIT takeaway

Every test should answer a question.

A strong diagnostic plan turns a theory into a series of logical decisions. Each result should either confirm something, eliminate something or determine what happens next.

Once testing has identified the failure, however, the investigation isn’t finished.

In the next article, we’ll explore ‘Determine the Cause’; moving beyond what has failed to understand why it failed.

Because finding the failed component might fix today’s symptom.

Finding the cause helps prevent tomorrow’s comeback.

To download a copy of the FIXDIT guide, and explore the framework in more detail, simply follow this link.