You Disagree With the Last Tech's Diagnosis: Decision Tree
Why this matters
You walk into a job the last person already touched - maybe a coworker, maybe a shop the customer used before - and what you find does not match what they wrote. The temptation is to declare the prior tech wrong and start over. Sometimes that is right. Often the prior diagnosis was correct for what they saw, and the system has changed, or you are looking at a second fault stacked on the first. Calling it wrong too fast burns a relationship and risks repeating their work for nothing. This tree keeps you honest about when to override and when to build on what is already there.
Start here: did the prior work create a hazard
Before you judge the diagnosis, check the state they left the equipment in. If the previous repair left something unsafe - a bypassed safety device, a loose connection, a line not properly sealed, a guard removed - make it safe before you do anything else. De-energize, relieve pressure, or shut off as the hazard requires, and do not return the equipment to service in an unsafe state just because someone else left it that way. A dangerous condition you inherited is still yours the moment you put hands on it. Fix the hazard first, sort out the diagnosis second.
For everything below, assume the prior work is safe and the question is purely whether the diagnosis was right.
First move: read their diagnosis charitably before you reject it
Assume the last tech was competent and had a reason. Your job is to find out what they saw, not to prove they were dumb.
- Get their actual notes or report, not the customer's summary of them. What did they measure, what did they replace, what did they recommend?
- Ask what the symptom was when they were here. Intermittent faults look different on different days. They may have diagnosed correctly for the condition present at the time.
- Check the date. A diagnosis that was right two months ago can be overtaken by a new failure. The system you are testing is not the system they tested.
If you cannot reconstruct what they were looking at, you are not yet in a position to say they were wrong - only that you see something different.
Second move: separate "wrong" from "incomplete" from "overtaken"
There are three very different reasons your finding differs from theirs, and they lead to different actions.
| Reason for the difference | What happened | Your move |
|---|---|---|
| They were wrong | Misread, mistested, or guessed | Re-diagnose from scratch; document why |
| They were incomplete | Right on one fault, missed a second | Build on their fix; add the missed item |
| They were overtaken | Right then, the system changed since | Note the timeline; this is a new fault, not their error |
| You are wrong | They were right; you misread | Re-check your own reading before you commit |
That last row is not optional. Before you override anyone, confirm your own diagnosis against a tool you trust. Disagreeing is not the same as being right.
Third move: reproduce the fault yourself, cleanly
Do not diagnose off their report or against it. Diagnose the equipment in front of you as if no one had been here.
- Measure the disputed thing yourself with a verified tool. A clean independent reading settles most of it.
- Confirm the symptom is present now. If you cannot reproduce the original complaint, you may be chasing a fault that is no longer there, or one that only shows under conditions you have not recreated (see related: the fault that tracks with usage).
- Look for a second fault. The most common reason a prior fix "did not work" is that it fixed a real problem and a different problem remained. Two faults stacked look like one wrong diagnosis.
Fourth move: decide override, build-on, or escalate
Now you choose your path, and how you say it matters as much as what you do.
- Override when your verified diagnosis clearly contradicts theirs and you can show the evidence. Document the new finding and what changed; do not editorialize about the other tech.
- Build on when they were partly right. Credit the part that was correct, add the part that was missed. This is the most common honest outcome.
- Escalate or get a tie-breaker when you and the prior tech are both confident and opposed, and the stakes are high. A third reading or a senior opinion beats two people digging in (see related: the tie-breaker when two pros disagree).
However it resolves, write down what you found and why, in evidence terms. If the customer or the other shop reads it later, it should sound like a measurement, not a grudge.
Recap
- Make any inherited hazard safe before you evaluate the diagnosis.
- Read the prior diagnosis charitably; get their actual notes and the symptom they saw.
- Sort the difference into wrong, incomplete, overtaken, or you-are-wrong - and confirm your own reading first.
- Reproduce the fault independently with a trusted tool, and look for a stacked second fault.
- Override, build on, or escalate - and document in evidence terms, not blame.
The judgment to keep: "different from what they wrote" is a reason to look harder, not a license to declare yourself right. Let the equipment, measured cleanly, make the call.
References
- Trade-standard practice for diagnostic documentation and handoff
- Manufacturer documentation for component test criteria
- OSHA guidance on inspecting equipment and correcting hazardous conditions
- See related: The Second Opinion the Customer Got (decision tree); The Tie-Breaker When Two Pros Disagree