Diagnosis, Repair, and Customer Service
The final domain tests how you think and how you communicate. Questions present a complaint and ask for the next diagnostic step, the correct documentation, or the right thing to tell a customer. The exam rewards a repeatable process and penalises parts-swapping.
The diagnostic process
- Verify the complaint in the customer's own terms, then reproduce it. Record the conditions: ambient temperature, load, water level, recent work, and any aftermarket accessory.
- Gather information: service manual, wiring diagram, technical service bulletins, stored fault codes, and prior repair orders.
- Isolate by dividing the system and testing one half at a time. Test components under load, not only with an ohmmeter.
- After repair, verify under the same conditions that produced the complaint.
- Document the complaint, cause, correction, parts, measurements, and any declined work.
Electrical diagnosis
- On high-current circuits, voltage drop testing beats resistance testing. Measure from source to load while the circuit is loaded; more than about 0.5 V on a 12 V circuit warrants investigation.
- Check chassis grounds first on any 12 V complaint. They corrode and produce faults that look like failed components.
Customer service
- Obtain authorization before work exceeds the estimate, and explain the fault in plain language alongside the price.
- Separate OEM warranty, extended service contract, and customer-pay work. Document customer-supplied parts and modifications.
- Never promise a completion date that depends on a parts supplier you do not control.
Worked example
Complaint: the 12 V lights dim when the furnace starts. Verify whether all circuits dim or only one; here, all of them. Check rested battery voltage at 12.1 V, which suggests about 50 percent state of charge. Load test: 11.4 V at the battery and 11.2 V at the distribution panel, a 0.2 V drop, which is acceptable. The wiring is therefore sound and the battery is the cause. Recommend charging and load testing, then replace if it fails, and confirm converter output near 13.6 V. Reporting a loose ground without measuring would have been a guess.
Traps
- Shotgunning parts instead of isolating the fault.
- Returning a unit that works in the shop but not under the customer's conditions.
- Failing to record declined recommendations, which later becomes a liability.
- Trusting an ohmmeter reading on a corroded connection that shows continuity but cannot carry current.
- Quoting a repair before confirming the diagnosis.