Error S.36

Safety first: Do not work inside the boiler or touch internal wiring unless you are a qualified engineer. Isolate mains and gas before opening the appliance. If you smell gas, evacuate and call the gas emergency number immediately. Working on gas appliances without Gas Safe registration is illegal and dangerous.

Initial homeowner checks you can do safely:

1) Note exactly what the display shows and when the S.36 appears (steady code, intermittent, after a power cut, after changes to controls).

2) Check basic power: ensure the boiler isolation switch (by the boiler) is ON and no fuses or breakers in the consumer unit have tripped. Check other household appliances to confirm mains supply is present.

3) Check the room thermostat and programmer: confirm the heating is set to ON/Manual call for heat and the target temperature is above room temperature. If the thermostat is battery powered, replace the batteries.

4) Try a boiler reset (using the boiler controls) once. If the code returns immediately or within a short time, this indicates a persistent fault.

5) Check boiler pressure gauge (if applicable) — low water pressure can cause other errors; top up only if you know how and it is safe to do so.

If basic checks do not restore heating, proceed with these diagnostic steps (only if you are competent with simple electrical checks; otherwise call a professional):

6) Confirm whether domestic hot water still works. If hot water is fine but CH is off, that points toward a heating demand/control issue rather than total boiler failure.

7) Check wiring to external controls: for older/simple setups some boilers expect a link across specific terminals (Vaillant docs reference terminals 3 and 4 for CH demand). If you have no external controls fitted the link may be removed; if you are unsure do not fit links yourself — get an engineer.

8) If you have basic electrical skills and a multimeter, you can check for the presence of the correct control voltage at the boiler control terminals while the thermostat/programmer is calling for heat. Only perform this if you know how to work safely with live circuits. If there is no control voltage, the problem is in the thermostat/programmer wiring or their power supply.

9) If the boiler intermittently loses demand or S.36 correlates with dips in household voltage, consider mains voltage problems or an internal low-voltage transformer fault — this will need an electrician and a Gas Safe engineer to inspect and possibly replace the PCB or transformer.

10) Check for other fault codes displayed alongside S.36 or in the boiler fault memory — codes relating to flow/return NTC sensors, eBUS or PCB faults change the diagnosis and usually require engineer intervention.

When to call a professional and what to tell them:

- Call a Gas Safe registered engineer (and an electrician if you suspect mains voltage issues) if the simple checks above do not clear the code or if you are not confident performing electrical checks. Do not attempt to open the boiler or replace internal components yourself.

- When you call, give the engineer: the exact boiler model, the S.36 code and any other codes shown, whether hot water works, whether the fault started after a power cut or new thermostat installation, and what simple checks you have already done. This helps them diagnose faster.

Summary: S.36 usually points to a control/voltage/demand signal problem. Start with safe, simple checks (isolation switch, programmer/stat settings, batteries, reset). If the fault persists, stop and get a Gas Safe engineer and/or qualified electrician to diagnose and repair internal wiring, PCB, or transformer faults.