Boiler showing a fault code or just gone quiet? Get it diagnosed, not guessed at

No heating or hot water in a household with young children, someone elderly or someone unwell is treated as urgent, and so is a boiler that is leaking, making an unfamiliar banging or has tripped the electrics. Say what the display shows and what the boiler is doing, and you are given an honest time for the visit. A boiler that has locked out because the pressure is low can often be got going over the phone by topping up the system, and if that is all it needs, you are told so rather than sent an engineer.

If there is a smell of gas, soot around the case or a pilot flame burning yellow and lazy, switch the boiler off, open windows and call the National Gas Emergency number on 0800 111 999 before anything else.

Diagnosis starts with the boiler's own memory. Every modern boiler logs its faults, and the sequence of codes over the last few days usually tells the story before the case is off. From there it is measurement: gas pressure and flue gas readings for combustion problems, resistance and voltage tests for sensors, fans, pumps and valves, pressure and flow checks for the water side. The component that fails the test is the one that is replaced.

The van carries the parts that fail most: fans, pumps, diverter valves, pressure sensors, ignition electrodes and leads, expansion vessels, automatic air vents, PCBs for the common makes, printed circuit boards for the common makes, condensate traps and gaskets. Where the boiler is a less common make or the part is model-specific, it is ordered, and the boiler is left safe in the meantime.

Low pressure lockouts come from slow leaks or a flat expansion vessel. No hot water but heating works on a combi is usually the diverter valve or a scaled-up plate heat exchanger. Heating works but the radiators stay cold is more often a system fault, a stuck zone valve, a failed pump or a sludged system, than the boiler itself. Ignition faults come from electrodes, gas valves, fans and flue problems. Kettling, a rumbling as water boils on hot spots, means scale or sludge in the heat exchanger. Fan faults are bearings wearing out, often heard as a whine before the boiler stops. Frozen condensate pipes shut boilers down in cold snaps and are cleared with warm water and then re-routed or insulated so it does not happen again.

The price for the visit and the diagnosis is agreed when you book, and the price for the repair is agreed once the fault is confirmed, before the part goes in. Where a repair is done on the first visit, the parts and the labour are on one invoice with no additions. Where a return visit is needed, that is quoted before the part is ordered. If the honest advice is that the boiler is not worth repairing, you are told that with the diagnosis, and you owe the diagnostic price only.

Repairs use manufacturer parts or manufacturer-approved equivalents, because a pattern fan or a cheap PCB fails early and takes the labour cost with it. Every replaced part is left for you to see if you want it. Where a part is obsolete, which happens on boilers over fifteen years old, you are told which part, whether a reconditioned unit is available and what that means for the boiler's future.

Any repair that opens the combustion side of a boiler finishes with a flue gas analysis to confirm the burner is combusting correctly, and any repair on the gas side finishes with a tightness test. The readings are recorded on the job sheet. If a repair reveals something that makes the boiler unsafe, it is classified using the industry procedure and made safe, and you are told what is needed to put it back into use.

Worcester Bosch, Vaillant, Ideal, Baxi, Viessmann, Glow-worm, Potterton, Alpha, Intergas, Ferroli, Vokera, Main, Ravenheat, Saunier Duval and the rest. Combi, system and heat-only boilers, open-vented and sealed systems, and the back boilers that still heat a surprising number of London terraces. The engineers carry the fault-code references and the service manuals for each, and the diagnosis is done to the manufacturer's procedure rather than from memory.