Error S.O

Safety first: do not open the boiler casing or attempt gas or electrical repairs unless you are a qualified Gas Safe engineer. Isolate electrical supply at the fused spur only if you intend to leave the boiler off; do not touch internal components. If you smell gas, leave the property immediately and call the gas emergency number.

Initial homeowner checks you can safely perform:

1) Check the programmer/timer: ensure the central heating channel is set to ON or the current time is inside a heating period. Some programmers have separate Holiday or Off modes. Reset to a normal heating schedule.

2) Check the room thermostat: set the thermostat to a temperature higher than the current room temperature so it should be calling for heat. If you have a wireless thermostat, check its batteries.

3) Check radiator TRVs: make sure TRVs are not closed on most radiators (one closed valve can be fine, but if many are closed the system may not call).

4) Check boiler mode: ensure the boiler is not set to summer mode or DHW-only mode. On Vaillant controls check that central heating is enabled.

5) Check boiler display and LEDs: note any other fault codes or messages. If only S.O (no heat demand) is displayed and the controls are asking for heat, record exact message and time.

6) Check electrical supply and isolation switch: verify the external fused spur to the boiler is switched on and the mains supply to the house is live. Do not open the boiler to inspect internal fuses.

7) Check system pressure: glance at the pressure gauge—low pressure can prevent normal operation. If pressure is very low (well below 1 bar) the boiler may not operate properly. Do not attempt to add water unless you are comfortable with your system’s filling loop procedure; otherwise call an engineer.

If you have completed the above and the boiler still shows S.O while you know the thermostat/programmer is calling for heat, do not attempt internal repairs. Next diagnostic steps (for a qualified engineer):

1) Verify a live CH call at the boiler terminals: the engineer will measure the voltage on the central heating call terminals to confirm whether the thermostat/programmer is sending a call for heat.

2) Check external control wiring and connectors: continuity and correct wiring to the PCB, including wireless receiver connections, room stat wiring and programmer wiring.

3) Inspect relay and PCB behaviour: confirm that the PCB is receiving the call and that the relay output is functioning. Check for stored fault history in the boiler’s system log.

4) Check for other component faults that can prevent recognition of demand: motorised valve position (if fitted), pump operation, fan or other safety interlocks. Faulty components or a blown fast-blow fuse on the PCB can cause symptoms where the boiler cannot accept or respond to a heat demand.

5) Check flow/return NTC sensors and related circuitry: if sensors or harnesses are defective or show extreme/unexpected values the control logic can inhibit operation; test resistance and wiring to confirm.

6) Replace or repair faulty parts as indicated: PCB, relays, wiring harness, thermostat, motorised valve, pump or sensors. Any component replacement and internal electrical work must be done by a Gas Safe engineer.

When to call a professional: if simple checks (thermostat, programmer, mode, power) do not restore heating, book a Gas Safe registered engineer. Tell them the boiler shows S.O (no heat demand) but that you have a confirmed call for heat from the thermostat/programmer. Provide any other codes you see and whether hot water is affected. The engineer will perform safe internal diagnostics and repairs, test the system, and advise on any component replacements.

Notes and caution: do not attempt internal fuse replacement, PCB work, sensor replacements, or gas appliance repairs yourself. Avoid repeated resets as this may mask an underlying fault. A qualified engineer should carry out further testing and any remedial action to ensure safe, correct operation.