Error .15

Safety first: if you are not a competent person for electrical or gas work, stop and call a Gas Safe registered engineer. Do not work on gas components. Before any inspection turn off the boiler electrically at the isolator and, if you will access internal components, switch off the gas supply at the appliance or meter. Allow the boiler to cool and depressurise if it has recently been running.

Initial homeowner checks you can do without opening the boiler:

1) Note and record the displayed code and any other messages. Attempt a single reset using the boiler reset button; if the fault returns immediately or repeatedly, do not keep resetting. 2) Check central heating pressure on the boiler gauge; top up to the normal operating pressure (usually ~1–1.5 bar) only if you know how and the boiler instructions permit it. 3) Check for obvious water leaks around the boiler or frozen/blocked condensate pipe which could cause related faults. 4) Ensure the electricity supply and gas supply to the property are working (try other appliances) and check the external condensate and flue for blockages.

If you are competent with basic electrical testing and choose to proceed (and the boiler is isolated):

1) Remove the boiler outer casing following the manufacturer’s instructions. Keep the appliance powered off and gas isolated while you open the cover. 2) Locate the flow and return NTC sensors (the guidance for Vaillant models shows flow on the left (hot) and return on the right (cold); sensors are small thermistor plugs clipped to the flow and return pipes and plug into the PCB). 3) Visually inspect sensor plugs and wiring for loose connectors, corrosion, crushed or chafed cables, or signs of water ingress. Reseat any loose plugs firmly. 4) Using a multimeter, measure the NTC resistance at the sensor plug and at the plug on the PCB wiring harness if accessible. At ~20°C an NTC for many Vaillant boilers should be around 10–12 kΩ (consult the exact sensor spec for your model). An open circuit (infinite/OL) or a short (very low ohms) indicates a faulty sensor or wiring. Also check continuity of the wires back to the PCB to confirm no breaks. 5) If a single sensor shows open/short or an incorrect fixed value, replace that sensor. Vaillant guidance and practical trade advice often recommends changing both flow and return sensors at once to make diagnosis quicker; replacing both removes the variable of swapping sensors and is a common first-line fix. On many models the sensors push-fit on the pipe and unplug from the wiring loom—replace with correct OEM/compatible NTC sensors and ensure the colour-coded connections are correct.

After replacing sensors or reconnecting plugs:

1) Reassemble the cover, restore gas and electrical supply, repressurise and bleed radiators if needed, then run the boiler and observe. 2) If the code clears and the boiler runs normally, monitor for recurrence and any signs of wiring or moisture ingress that could cause repeat faults.

When to call a professional and escalation:

1) Call a Gas Safe registered engineer if you are not confident performing the checks, if the fault persists after reseating or replacing sensors, if wiring to the PCB appears damaged, or if you suspect a PCB/electronics fault. 2) If measurements show correct sensor resistances but the boiler still faults, do not attempt PCB repair yourself; this requires an authorised engineer. 3) If you find evidence of internal water leaks, pump failure, overheating or dry-fire safety trips, get the boiler isolated and call a Gas Safe engineer immediately.

Summary note: .15 is an NTC-type fault. Start by checking connectors and wiring and consider replacing the flow and return NTC sensors as a primary diagnostic step, but involve a Gas Safe engineer for any gas, mains electrical, or PCB-level work or if the problem is not resolved by sensor replacement.