Find evidence: The foundation of every successful diagnosis
27 July 2026 
In the first of a series of articles for Aftermarket Magazine, Darren Quinn from Mechanic Mindset takes us through his ‘FIXDIT’ framework, designed to provide a simple, memorable way to apply the critical stages required for a successful diagnostic routine. This month, we explore the first stage, ‘Find evidence’, with future issues set to cover the subsequent stages: ‘Inspect evidence’, ‘eXecute diagnostic plan’, ‘Determine the cause’, ‘Implement repair’, and ‘Test’.
Successful diagnostics has never been about luck. The best technicians follow a process.
Whether diagnosing an intermittent electrical fault, a drivability issue, or a complex network communication problem, the same fundamental stages must be followed. The challenge is that many technicians don’t consistently apply them. Steps are skipped, assumptions are made, and testing begins before the problem is fully understood.
So it is essential to start with the foundation of every successful diagnosis: ‘Find evidence’.
Before testing begins
When a vehicle arrives with a fault, most technicians feel an immediate urge to start testing. Scan tools come out, fault codes are checked, and measurements begin. It feels productive. It feels like diagnostics.
But in many cases, the biggest diagnostic mistake occurs before a single test lead is connected.
The mistake is failing to gather enough evidence.
Modern vehicles are incredibly complex. A single customer complaint may involve multiple control modules, network communications, software strategies, environmental conditions, and intermittent operating states. If technicians begin testing without first fully understanding the problem, they risk heading down the wrong path from the very beginning.
At Mechanic Mindset, we teach that diagnostics is not a process of testing first. It is a process of investigation first.
Start with questions
One of the most valuable sources of evidence is standing right in front of you: the customer.
Unfortunately, this is often the most overlooked part of the diagnostic process. In a busy workshop, conversations can become rushed. Customers may be interrupted, assumptions are made, and important details are missed.
Professional technicians understand that customer questioning is not administration. It is evidence gathering.
Consider two different approaches.
“Does the engine warning light come on all the time?” Or. “Can you explain exactly what happens when the problem occurs?”
The second question encourages the customer to provide context, timing, conditions, and observations that may never emerge from a simple yes-or-no answer.
Intermittent faults particularly depend on this information. A customer may reveal that the fault only occurs after motorway driving, during heavy rain, when towing, or on cold mornings. These details can dramatically change the direction of the diagnosis.
The goal is not simply to hear the complaint. The goal is to understand the circumstances surrounding the complaint.
The vehicle has a story
The second source of evidence is the vehicle itself.
Before reaching for advanced test equipment, technicians should perform a thorough initial inspection and complete a function test. Many faults leave clues long before any measurements are taken.
A full function test is about far more than simply confirming the customer’s complaint. Professional technicians assess all related systems and record both what is working and what is not. This is important because systems that operate correctly can often be just as valuable as those that have failed.
For example, if several electrical systems are inoperative while others continue to function normally, that information can help identify common power supplies, grounds, modules, or network pathways that remain unaffected. Every observation helps build a clearer picture of the fault.
Alongside the function test, technicians should carry out a careful visual inspection of the vehicle. Signs of water ingress, damaged wiring, corrosion, loose connectors, aftermarket modifications, fluid contamination, and physical damage can all provide valuable evidence.
Experienced technicians also learn to engage all of their senses during this stage. An unusual noise, an abnormal vibration, a burnt smell, or signs of moisture inside the vehicle may all point towards the conditions that contributed to the fault.
The more evidence collected during this stage, the easier it becomes to identify patterns, establish direction, and build an effective diagnostic plan.
Fault codes are evidence, not answers
One of the most common mistakes in modern workshops is treating fault codes as diagnoses.
A fault code simply indicates that a control module has detected something outside of its expected operating parameters. It does not automatically identify the failed component.
Yet many technicians still fall into the trap of reading a fault code and immediately replacing the part it names.
Professional diagnostics requires a different mindset.
Fault codes, freeze-frame data, event histories, live data values, and network information are all forms of evidence. They help build a picture of what the vehicle is experiencing, but they should never be viewed as the final answer.
Another common mistake is clearing fault codes too early.
Once evidence has been erased, it may never be recoverable. Freeze-frame data that could have identified operating conditions at the moment of failure may be lost forever.
Define the problem
The purpose of the ‘Find evidence’ stage is simple: accurately define the problem.
Many diagnostic failures occur because technicians attempt to solve a fault they have not fully understood. If the initial complaint is poorly defined, every stage that follows becomes more difficult.
The technicians who consistently solve difficult faults are rarely the ones who rush into testing first.
They are the ones who gather the best information. They ask better questions. They observe more carefully. They preserve evidence. They build a complete picture before making decisions.
In the next article, we’ll explore the second stage of the FIXDIT process: ‘Inspect evidence’. This is where technicians begin analysing the information they have gathered, identifying patterns, eliminating distractions, and turning evidence into a logical diagnostic direction.
Click here to download a copy of the FIXDIT guide, and explore the framework in more detail.