Error S.31

Safety precautions:

- If you smell gas, evacuate the property, do not operate electrical switches, and call the gas emergency number immediately.

- If there is visible water leaking from the boiler or pipework isolate the mains water to the property (if you know how) and switch off the boiler electrical supply, then call an engineer.

- Before opening the boiler casing or touching internal wiring, switch off the boiler electrical supply at the isolator or consumer unit and only proceed if you are competent and comfortable working with mains electricity and low-voltage thermostat wiring.

- Do not attempt any gas appliance repairs or internal combustion component work yourself. Only a Gas Safe registered engineer should work on gas, flue, burner, mains gas valves or internal PCB issues.

Initial homeowner checks you can do (no tools required):

1. Check the room thermostat knob or digital thermostat: ensure it is switched to HEATING (not OFF) and the set temperature is above the current room temperature. For dial thermostats turn knob up; for digital thermostats set the mode to HEAT and raise target temperature.

2. Check the programmer/timer/central control: confirm the heating channel is enabled for the current time and day (not in an OFF period or holiday mode). If you have separate CH and HW programming, ensure CH is active.

3. Check any wireless thermostats: replace batteries and confirm the thermostat is showing as active and connected.

4. Check TRVs (radiator thermostatic valves): if TRVs are closed on all radiators they can prevent a heat demand; open a radiator TRV slightly and see if the boiler accepts demand.

5. Look at the boiler display for other codes: if S.31 is shown alone it means no demand; other fault codes would indicate separate issues that need repair.

6. Try a basic reset: press and hold the boiler reset button (often a flame symbol with a line) for a few seconds. Note: resetting is usually unnecessary for S.31 but can clear transient conditions; do not repeatedly reset if a fault repeatedly appears.

If CH controls are on but boiler still shows S.31 (diagnostics you can perform if competent):

1. Confirm wiring/terminals: with the boiler powered but heating call off, you can call for heat from the thermostat and observe the boiler display. If you are competent with a multimeter, you can measure across the CH call terminals at the boiler (often labelled SL, CH, or a dedicated thermostat input according to the manual). A closed contact or appropriate control voltage should appear when the thermostat calls for heat. If there is no change at the boiler terminals when the thermostat calls, the problem is upstream (thermostat, programmer, or wiring).

2. Wireless thermostat checks: verify receiver module (if fitted) is powered and indicator shows a call when you press the thermostat; replace batteries or consult the thermostat manual for pairing status.

3. Programmer override: use the programmer’s manual override to turn CH on manually and see whether the boiler then accepts the demand.

4. External controls and interlocks: check any additional controls (room controllers, zone valves, external relays, smart controls) that sit between the thermostat and boiler. A stuck or failed relay/actuator or a faulty zone valve end switch can prevent the boiler receiving a call.

When to call a professional and what to tell them:

- Call a Gas Safe engineer if: you cannot get the boiler to accept a legitimate heating request despite confirming the thermostat and programmer are set correctly; you suspect wiring or PCB faults; you find damaged wiring, burning smells, water leaks or other fault codes besides S.31; or you are not confident carrying out electrical checks.

- Tell the engineer the exact display code (S.31), what you have already tried (thermostat set to heat, batteries changed, programmer override, reset), whether DHW is working, and whether radiators or zone valves are responding. That information will help them diagnose quickly.

Notes and cautions:

- S.31 is a status code indicating no heating demand, not a boiler internal fault in itself. Often the fix is simply restoring the thermostat/programmer to demand heat.

- Do not attempt to repair gas, flue, burner or PCB components — always use a Gas Safe registered engineer for those.

- If you are unsure about electrical checks, stop and call a professional rather than risk electric shock or further damage.