Maintenance, Fault Diagnosis and Repair
This domain tests diagnosis: how you find a fault logically and prove the repair.
Method
Gather the complaint, reproduce it, check the simple things first, then test in sequence - pressure, flow, temperature, electrical continuity, control response. Isolate before working, and re-test after the repair. Write down readings so the next visit starts from evidence.
Common faults
- Running cistern: inlet valve or a distorted flush valve seal.
- Dripping tap: worn washer, ceramic cartridge, or scale on the seat.
- Low combi pressure: system leak, failed expansion vessel, or a passing relief valve.
- No hot water: diverter valve, cylinder stat, zone valve or air lock.
- Gurgling or smells: lost trap seal or a failed AAV.
- Frozen condensate: boiler locks out in frost; thaw and reroute the pipe.
- Sludge and cold spots: corrosion products; flush and treat.
Worked example
A combi loses pressure every week and the relief pipe outside is wet. The expansion vessel has lost its charge, so heating expansion lifts the relief valve. Check the vessel charge with the system depressurised; if water comes from the Schrader valve, the diaphragm has failed and the vessel is replaced. Repressurising alone would not fix it.
Traps that cost marks
Replacing parts without testing; ignoring the relief discharge; not checking expansion vessel charge before the system pressure; failing to record readings and the repair.