Error F5

Safety precautions:

- If you smell gas, evacuate the property immediately and call the gas emergency number — do not attempt any checks.

- Do not open the boiler casing or attempt to replace internal components unless you are a Gas Safe registered engineer. Working on gas appliances or exposed wiring is hazardous and illegal for unqualified people.

- Turn off mains power to the boiler before doing any external electrical checks.

Initial checks a homeowner can do (safe, non-invasive):

1. Note the fault code and any accompanying lights or messages. Record when it occurred and whether heating/hot water was running.

2. Check boiler and system pressure on the gauge. Cold system pressure should normally be around 1.0–1.5 bar. If pressure is very low, top up the system using the filling loop only if you are confident how to do this safely and the manual permits it.

3. Check all external controls: programmer/timers, room thermostat and cylinder thermostat are calling for heat and set correctly. Ensure the boiler mode is ON (per the S30 control knob) and time settings are correct.

4. Power-cycle/reset: use the boiler’s Restart/reset button first. If that does not clear the fault, switch the boiler off at the mains for 30–60 seconds, then restore power and try restarting. Some transient sensor faults clear with a reset.

5. Look for obvious external problems: frozen or visibly damaged pipes, recent water leaks, or recent electrical disturbances (power cuts) that preceded the fault.

Further diagnostic steps (what an engineer will do or you can report to them):

1. If the fault persists after reset, prepare to call a Gas Safe engineer. When you call, give the engineer the boiler model (Keston S30) and the F5 return thermistor fault and tell them which checks you have already tried (reset, pressure reading, any recent faults or freezing).

2. The engineer will typically check the wiring and connector for the return thermistor (looking for loose, corroded or damaged connections) and measure the thermistor’s resistance with a multimeter to confirm if it is open, shorted or giving an implausible temperature reading. They may also check the flow thermistor and compare readings to ensure the flow/return differential is reasonable.

3. If wiring and connectors are good but the thermistor is out of specification, the thermistor will be replaced. Replacement requires access behind the boiler cover and possibly draining part of the system, so it must be done by a qualified engineer.

4. If the thermistor tests OK, the engineer will investigate the PCB and sensor input circuitry and look for a control board fault or software/configuration issue. They will also check for reversed flow/return connections where relevant and rule out flow problems or pump issues causing abnormal temperature differentials.

When to call a professional now:

- If the fault remains after a reset or recurs frequently.

- If you are not comfortable topping up pressure or if pressure drops repeatedly.

- If you find damaged wiring or connectors to the boiler.

- Any time the boiler requires opening, internal testing or replacement of sensors or PCB components.

What to tell the engineer when they arrive:

- Boiler model and serial if available (Keston S30).

- Exact fault code (F5) and any other display information.

- What checks you have done (reset, power-cycle, pressure reading and value, any obvious external issues).

- Any recent events (service, power cut, freezing, water leaks).

Summary: Try a safe reset and basic external checks first. If F5 remains, do not open the boiler — call a Gas Safe registered engineer to test/replace the return thermistor or investigate wiring/PCB faults.