You Have Two Plausible Causes and One Visit: Decision Tree
Why this matters
The symptom fits two causes equally well, you have one truck roll to settle it, and guessing wrong means a return trip, a wrong part, or a customer who watched you fix the thing that was not broken. The skill here is not knowing more, it is choosing the one measurement that separates the two theories fastest. A tech who can name the discriminating test resolves in one visit what a parts-swapper drags across three.
Start here: write down both causes as testable claims
Before you touch a tool, state each theory as something a reading can confirm or kill. Vague theories cannot be discriminated.
- Bad: "it is probably the control or the sensor."
- Good: "if it is the sensor, the controller sees a wrong value while the real value is fine; if it is the control, the value is right but the output does not follow."
Now you have two claims that predict different readings at the same test point. That is the seam you split.
Branch 1: is there one test that separates them
Look for the single measurement where the two causes predict opposite results. This is the discriminating test, and it is worth more than three tests that both causes pass.
- Electrical no-output: upstream cause versus device cause both look identical at the load. Measure at the device terminals with the circuit calling. Voltage present but no output points to the device. Voltage absent points upstream. One reading, both theories tested.
- HVAC weak cooling: low charge versus poor airflow both drop capacity. Airflow is the discriminator, so check it first (a clean filter and a measured temperature split), because a low split with good airflow points to charge, while good split with low airflow points to the air side.
- Plumbing low flow at one fixture: supply restriction versus fixture restriction. Test upstream of the fixture. Full flow at the stop with weak flow at the outlet isolates the fixture.
- Pool poor circulation: pump versus blockage. Read the filter pressure. High pressure with low return flow means a downstream restriction; low pressure with low flow means the pump is not moving water.
Branch 2: order by the cost of being wrong
If no single test cleanly separates them, run the cheaper theory to disprove first, but weight it by what a wrong guess costs, not just test time.
- Run the reversible test first. A test that damages nothing and can be undone beats one that commits a part.
- Weight by consequence. If one cause is a safety item (a failing protective device, a gas or water leak) and the other is a comfort complaint, chase the safety cause first even if it is the less likely one. A missed hazard is not a fair trade for saving a test.
- Weight by access. If one theory needs a system shut down the customer will not release right now, test the other theory first and leave the shutdown-required test as the tiebreaker you schedule.
Branch 3: if both tests come back ambiguous
Sometimes both readings sit in the gray zone. Do not force a call.
- Load or stress the system and re-read. Faults separate under load that hide at idle.
- Compare against a known-good reference: an identical circuit, the other side of a split system, the manufacturer value.
- If it is still ambiguous and no hazard forces your hand, monitor with the least-committal action and say so plainly rather than swapping a part on a coin flip.
When to pick which approach
| Situation | Approach |
|---|---|
| A clean discriminating test exists | Run it first; it settles both at once |
| No single test, low stakes | Disprove the cheaper, reversible theory first |
| One cause is a safety item | Chase the hazard first regardless of odds |
| One test needs a shutdown you cannot get | Test the other theory now, schedule the shutdown |
| Both readings ambiguous | Load the system, compare to a reference, or monitor honestly |
The recap
- Turn both causes into claims that predict different readings.
- Find the one test where they predict opposite results and run it.
- With no clean discriminator, disprove the reversible theory first, but let safety and access override the odds.
- If still ambiguous, stress-test, compare to a reference, or monitor - do not swap on a guess.
References
- Trade-standard differential-diagnosis practice (isolate the variable that separates causes)
- Manufacturer service literature for known-good reference values
- See related: Which Test First - Cheapest vs Most Diagnostic Sequencing; Narrow by Elimination vs by Most Likely First