Overview
FLASH OFF ON on a Baxi Solo indicates an overheat lockout. The boiler has detected that water in the primary circuit has reached an abnormally high temperature or that there is poor circulation, so the control has shut the burner down to protect the appliance and the system. This is a protective safety condition rather than a normal operating message. Common reasons for an overheat lockout are trapped air in the heating system (restricted flow in radiators or the heat exchanger), a seized or failed circulating pump, closed or partially closed valves, low or incorrect system pressure, or a failed temperature sensor/thermistor. In some cases scale or sludge in the heat exchanger can also reduce flow and cause overheating. The severity is medium to high because the boiler will not provide heating or hot water while locked out and continued attempts to run a boiler that is overheating can cause damage. Some initial, non-intrusive checks and a reset are suitable for a competent homeowner, but if the lockout recurs, if you find the pump is not running, if system pressure is unstable, or if there is any gas smell or electrical fault, you must call a Gas Safe registered heating engineer. Do not open the boiler casing or attempt gas or complex electrical repairs yourself.
Possible Cause: Overheat lockout
Troubleshooting Steps
Safety precautions:
- If you smell gas, stop, get everyone out of the property and call the gas emergency number immediately. Do not operate electrical switches.
- Before touching the boiler controls isolate the electrical supply at the wall switch and allow the appliance to cool to avoid scalding. Wear gloves and eye protection when bleeding radiators.
- Do not remove the boiler cover or attempt internal repairs unless you are a Gas Safe registered engineer.
Initial checks a homeowner can do:
1. Note the exact display pattern and any LEDs lit. Take a photo for the engineer if needed.
2. Check for obvious issues: are all radiator thermostatic valves open, is the room thermostat set to call for heat, and is the programmer/timer asking for heat?
3. Check the system pressure on the boiler gauge when cold. Typical cold pressure is around 1.0 to 1.5 bar for most domestic systems. If the pressure is very low or zero, topping up may be required (see below). If pressure is much higher than the recommended figure for your manual, do not top up and call an engineer.
Reset procedure (try before further checks):
- If your boiler has a dedicated reset button press and hold for a couple of seconds until the red lockout light or error clears.
- If your model uses the thermostat control knob, turn it fully anticlockwise and hold until the red lockout light goes out, then return to your desired setting.
- If it uses an ON/OFF/SELECTOR switch turn the selector to R (Reset) and hold for about 3–5 seconds then release.
After reset, allow a few minutes to see if the burner relights and the boiler starts to heat. If the boiler relights and runs normally, monitor for recurrence.
Bleeding radiators and restoring circulation:
1. Turn the heating off and allow the system to cool slightly to reduce scald risk, or follow the manufacturer guidance for bleeding while warm if you are experienced.
2. Use a radiator key and a small container to catch water. Open the bleed valve at the top of each radiator until air hisses out and water begins to run steadily, then close the valve.
3. Re-check system pressure and top up if necessary (see next step).
Topping up system pressure (only if pressure is low and you are confident to do so):
- Locate the filling loop on the boiler (or external filling loop) and follow the boiler manual: open the valve(s) briefly until the pressure rises to the recommended level (typically 1.0–1.5 bar cold), then close the valves securely. Do not leave the filling loop permanently connected. If you are unsure, stop and call an engineer.
Check the pump and flow:
- With the heating calling for heat, carefully feel the flow and return pipes near the boiler (do not touch hot surfaces directly). There should be a noticeable temperature difference and a gentle flow. Listen for the pump running (a quiet hum or vibration). If the pump is stone cold or not running and the boiler is calling for heat, do not attempt to open or strip the pump; call a Gas Safe engineer.
Other checks to make:
- Ensure motorised diverter valves (if fitted) are in the correct position and not stuck.
- Make sure TRVs and isolation valves on radiators are open so that water can circulate.
- Look for signs of leaks or water around the boiler; if present, isolate electrics and call an engineer.
When to call a professional (and what to tell them):
- If the boiler locks out again after a reset, or locks out immediately when you reset.
- If system pressure will not stay stable or you cannot top up the system yourself.
- If the pump is not running, noisy, or you suspect a circulation fault.
- If you see error codes other than the overheat indication, or if there is any sign of gas smell or electrical problems.
- When you call, tell the engineer the exact error display (FLASH OFF ON), what steps you have already taken (reset, bled radiators, pressure checked), the boiler model and serial number, and how frequently the fault occurs.
Final note:
A single reset and bleeding the system will often clear an overheat lockout caused by trapped air or low pressure. However persistent or repeating overheat faults usually indicate a component failure or restricted flow and must be diagnosed and repaired by a Gas Safe registered heating engineer. Do not attempt internal boiler repairs or gas or electrical work yourself.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Baxi Solo.