Reason from symptom to cause, work from the supply inwards, and learn the classic patterns: dead, short-cycling, kettling or sooting boilers.
Good fault-finding reasons from symptom to cause using the manufacturer's information, working from the supply inwards instead of swapping parts. Learn the method, then the classic patterns.
The method
- Gather evidence. Get a clear description of the symptom, read any fault code, and ask what changed recently, such as a power cut, a part swap or a loss of pressure.
- Follow the model's diagnostic path. Read the relevant fault-code entry, then work through the maker's checks in sequence and take the specified measurements. A code narrows the search, but it does not tell you which part has failed. A repair that worked before, or a routine service, is no guide to this fault.
- Split the system into areas: gas and electrical supply, the appliance, the controls and wiring, and the flue and condensate.
- Test from the supply inwards. Confirm the basics, such as supply present, system pressure correct and gas rate sensible, before combustion analysis or component tests.
- Change a part only when the evidence points to it, then prove the fix, confirm combustion and the safety devices, and recommission.
Never bypass or bridge out a safety device to get an appliance working: it guards against dangerous conditions.
Classic patterns
Completely dead (no heating, no hot water, nothing responds): start with the electrical supply and the controls. Confirm power is present and that the programmer and thermostats pass a demand through before opening the appliance. A low-pressure lockout would normally still show an indication.
Lock-out: the boiler has stopped on a fault and needs the cause checked against its own fault code and instructions. An anti-cycle delay ends as part of normal operation; an incorrect clock setting changes demand but does not create a safety lock-out. The display does not tell you the control board needs replacing.
Low-pressure lockout on a sealed system: many boilers will not fire below their minimum pressure. Verify the gauge or sensor indication, then check for draining, leakage or relief discharge. Restore the specified cold pressure as instructed and resolve the cause rather than repeatedly topping up.
Frequent short firings: check how the boiler is being controlled and whether heat reaches the system. Poor flow, minimum output above the active load, or an incorrect control or sensor signal can cause excessive cycling. With little room heating, start with the pump, valves, filters and circulation path. Some on/off cycling is normal when demand is below the appliance’s minimum modulation; short firings alone neither prove a part has failed nor clear the thermostat, pressure system or flue. Follow the model’s fault indications and measurements.
Repeated overheat lockout: the trip is protecting against a real fault. Diagnose it; resetting or turning the temperature down does not repair it.
Kettling (banging and rumbling as it heats): local boiling can follow impaired heat transfer or flow. Deposits on the exchanger are one likely cause; check circulation, air and water condition as well before deciding on treatment.
Yellow, lazy flames where a crisp blue flame is expected: incomplete combustion and possible carbon monoxide. Investigate the air supply, the burner and the flue.
Sooty marks near an open-flued boiler, with a smell of fumes: treat these as possible exposure warnings. Stop use and get people into fresh air; switch off and ventilate only if safe, without delaying getting clear. Carbon monoxide itself has no smell. From a safe place, report suspected gas-appliance CO to 0800 111 999. Seek urgent medical advice for possible symptoms: call 999 after suspected exposure with breathing difficulty, sudden confusion, loss of consciousness, weakness, or chest or muscle pain; for other possible CO symptoms, contact NHS 111. Stay out until advised it is safe to return. Once the immediate risk is controlled, use the specified combustion, air-supply and flue checks and classify the findings. Cleaning off soot or turning the burner down does not make it safe.
Pressure keeps falling on a sealed system: prove the fall before chasing it. A faulty gauge or pressure sensor can show a loss that is not there, so confirm it with a separate test gauge, comparing readings at the same temperature, because pressure rises as the water heats.
Air leaving a newly filled or commissioned system can lower the pressure even when no one has bled a radiator. Check that history as well as any recent draining or bleeding.
For a persistent, confirmed fall, investigate leaks, including concealed ones, and discharge through the pressure relief valve; look for water at the end of its discharge pipe. A relief valve that has discharged may be faulty, or it may have opened correctly because the pressure climbed too high when hot, which happens when the expansion vessel can no longer take up the expansion. Compare cold and hot pressures before condemning either part, and refill to the appliance's stated cold pressure, commonly about 1 to 1.5 bar, never to the 3 bar at which the relief valve typically opens.
Condensate lockout in freezing weather: a condensing boiler shuts down safely when its condensate cannot drain, and in a cold snap the usual cause is ice in a pipe run outside or through an unheated loft, garage or basement, often at the open end, a bend or a dip. A route through heated space to an internal soil stack or waste is the preferred design because it keeps the pipe out of the frost; a pipe in an unheated space indoors needs the same protection as one outside. Thaw a frozen section only where you can reach it safely from the ground, using warm, never boiling, water or a hot-water bottle, then reset the boiler by its instructions and review the route, pipe size and insulation.
Reading the branches of a fault chart
These examples use the Ideal Logic+ Combi2 C24/C30/C35, October 2023 instructions. They illustrate how measurements narrow a fault; use the instructions for the actual appliance before testing or changing a component. Electrical work needs appropriate competence, safe isolation and the specified precautions for any live measurement.
Temperature sensors and their wiring
A thermistor's resistance changes with temperature. With the sensor disconnected after safe isolation, compare the measured value with its actual temperature. This model's flow and return thermistor tables give:
- 25 °C: 9.7–10.3 kΩ.
- 60 °C: 2.4–2.6 kΩ.
- 85 °C: 1.0–1.1 kΩ.
A result outside the relevant band points towards the sensor. A correct result does not mean the wiring to it is sound. In the return-thermistor chart, check continuity through the harness next. An open path calls for checking and repairing or replacing the wiring; only a sound sensor and harness take that branch on to the control board.
Fan supply and pressure indication
In the fan-fault chart, check the connections and wiring continuity first. With those sound, the specified measurement across the blue and brown connections distinguishes two branches: 230 V AC present leads to the fan; that supply absent leads to the control board. Complete the prescribed checks and isolate safely before replacement. A fan-fault message does not mean the fan has failed.
For a low-water-pressure message, first establish that the boiler and heating system are filled and vented, their relevant valves are open, and the pressure is 1–1.5 bar. The next check is the pressure-switch connections. If those are secure and the earlier checks pass, the chart calls for a new water pressure switch. Adding more water when the actual pressure is already correct misses this diagnostic branch.
Overheat with a large temperature difference
After confirming the water content and open isolation, filter and radiator valves, compare flow and return temperatures. A difference greater than 30 °C directs the engineer to check that the circulating pump turns freely. The next branch asks whether the difference is below 20 °C. If the initial difference is not above 30 °C, the chart follows the thermistor checks instead. Follow that complete sequence; these are diagnostic thresholds for this model, not general targets for every system.
Follow the model's electrical and combustion precautions
For the ecoTEC plus VU/VUW ..6/5-5 covered by Vaillant's 2021 manual, the electrical sequence includes disconnecting every supply at all poles with the specified isolation arrangement, securing against reconnection, waiting at least three minutes for the capacitors to discharge, and checking that no voltage is present. The isolation contact gap is at least 3 mm, or the mains plug is removed where fitted. Waiting does not prove the circuit is dead: you still check for voltage.
During repair work on this Vaillant range, keep leak-detection spray or fluid off the Venturi filter's covering cap. Contamination can block the filter and destroy the mass-flow sensor. Carry out the required gas-joint leak checks by the specified method while protecting that component; spraying the cap is not a sensor test.
A gas family check showing Warning directs the engineer to the model's combustion procedure in P.01, where the specified venturi adjusting screw sets CO2. CO2 measurement is not possible during the gas family check itself. Use the correct firing rate, warm-up, casing condition and limits; never adjust on the strength of a fault message.
If the specified CO2 cannot be set, the diagnostic path checks that the gas restrictor is correct and undamaged: yellow for G20, blue for G25 and grey for LPG. Then repeat the gas family check as instructed. These colours belong to this model's parts; other appliances may mark theirs differently. If adjustment remains outside the specified range, do not start the appliance; refer to the manufacturer's service support. Resolve the fault and complete the required checks before leaving it in operation.
Gas appliances in a bath or shower room
Only a room-sealed gas appliance may be installed in a room with a working bath or shower, even one not built as a bathroom. The rule also covers a cupboard or compartment inside that room, and a space next to it with an air vent into it.
A room-sealed appliance has its combustion system sealed from the room: it takes its air from outside the room and discharges its products straight to open air. An open-flued appliance draws its combustion air from the room it stands in, and a flueless one lets its products mix with the room air.
An open-flued or flueless appliance found in a bath or shower room, a cooker included, is classed Immediately Dangerous under the current unsafe situations procedure. Explain why, then with the responsible person's permission disconnect it, seal the gas supply, attach a DANGER DO NOT USE label and complete a warning notice.