The Differential
Method.
The standard of care for diagnosis.
Version 1.0 · September 2026 · Sam Bruley
Eight steps for finding a fault and proving it, on any machine: a truck, a Harley, a loader, a furnace. It’s adapted from differential diagnosis in emergency medicine, where you can’t swap in a new heart to see if it helps. The rule underneath all eight: a conclusion is only as good as the elimination behind it.
The eight steps
- Confirm the complaint yourself. Reproduce it, or record that you couldn’t. Nothing starts from a description alone.
- Gather what’s already known, then distrust it. Codes, the last repair, what the customer says, what the last tech wrote. It’s all data; none of it is verified until you verify it.
- List what could cause it, and order the tests: cheapest first. A ten-second check before a two-hour one. The order is written down before the first test.
- Decide how each test runs: static or under the original conditions. A fault that only shows up hot, loaded, or at speed has to be tested hot, loaded, or at speed.
- Trace it. Electrical: trace the circuit and record what you measure against the specification. Mechanical: trace the failure (what moved, what didn’t, what it touched). Every reading goes on the record with its spec.
- Rule out the cheaper causes before naming the cause. No root cause is accepted while a simpler explanation is still standing. Until one thing is left, it isn’t found.
- Repair, then verify under the original conditions. The fix isn’t counted until the complaint from step 1 is gone in the conditions from step 4.
- Document the case. What was considered, what was ruled out and on what evidence, what was measured, what was done, what verified it. The record is the proof.
Two rules that don’t bend
Nothing leaves on a guess. A part is not replaced to see if it helps.
The record decides. Not the writer, not the tech’s confidence, not the customer’s patience. What was measured and what was ruled out.
Red herrings
A finding that looks like the cause but isn’t stays on the record as a red herring, separate from the things that were ruled out. It’s what the next tech, or the same tech next time, needs to know not to chase again.
What it doesn’t do
It doesn’t find the fault for you. It makes sure you don’t quit before you’ve proven it, and it won’t let you commit to the cheap wrong answer. It turns a good tech’s discipline into something every tech follows, and something the shop can see.
The comeback definition that goes with it is published separately: The Diaforik Comeback Standard.