Overview
The display pattern ON FLASH FLASH on a Baxi Solo commonly indicates a serious fault. On Solo models the manufacturer documentation lists the "flash on flash" pattern as a PCB/control fault, but the same symptom can also be triggered by upstream problems such as a circulation fault or a dry fire condition (the burner trying to run with little or no water flow). In short, it means the boiler has detected an unsafe operating condition and has locked out to protect itself. Severity: moderate to high. The boiler has shut down to prevent damage or overheating. If the underlying cause is a simple pressure or air-in-system problem you may be able to resolve it yourself. If the fault is actually a PCB or internal electrical/control failure, or if you smell gas, see water leaks, or the boiler has repeatedly locked out, you must call a Gas Safe qualified engineer. Repeated resets or attempting internal repairs without the right qualifications can make the situation worse and is unsafe. DIY or professional: start with basic homeowner checks (pressure, radiator bleeding, reset) but do not attempt electrical diagnostics, PCB replacement, gas valve work, or internal component repairs. If basic checks do not clear the lockout, or if the fault returns, call a Gas Safe engineer to diagnose and repair the boiler properly.
Possible Cause: Circulation error or dry fire
Troubleshooting Steps
Safety first
1. If you smell gas, evacuate the property immediately, call the gas emergency number and do not operate the boiler or any electrical switches. 2. If there are visible leaks, switch off the boiler at the external isolator and turn off the mains water supply to the heating system if safe to do so; call a professional. 3. If you have any doubt or the boiler has locked out repeatedly, do not continue troubleshooting—contact a Gas Safe engineer.
Initial checks a homeowner can do
1. Check the boiler display and note any full code or additional flashing pattern. Take a photo or write it down; this helps the engineer. 2. Check the central heating pressure gauge: aim for around 1.0–1.5 bar when cold on most Baxi systems. If pressure is below 0.5 bar the boiler will often lock out. 3. Check the room thermostat and programmer to make sure heating demand is present and timers/batteries are OK. 4. Listen at the boiler when a call for heat is made: can you hear the pump running (a quiet hum) or any unusual noises? 5. Inspect external flues/air intake for obvious blockages and check condensate pipe (in cold weather a frozen condensate can cause faults).
Basic corrective actions you can try yourself
1. Reset the boiler: locate the reset button (usually marked reset or a flame symbol) and press and hold for 5–10 seconds. Wait for the boiler to attempt restart. Do not repeatedly force resets; if it locks out again stop and get help. 2. If pressure is low: repressurise via the filling loop only if you are comfortable doing so. Open the filling loop valve(s) slowly until pressure rises to about 1.0–1.5 bar, then close the valve(s) tightly. After topping up, re-check for leaks and then attempt a single reset. 3. Bleed radiators to remove trapped air which can cause poor circulation. After bleeding, re-check system pressure and top up if it drops. 4. Check and clear condensate pipe if frozen (pour warm, not boiling, water over the external section to thaw). 5. If the pump appears not to be running and you can hear nothing when the boiler calls for heat, try to check that the pump isolation valves (if accessible externally) are open. Do not try to dismantle or electrically test the pump yourself.
Diagnostics and when to stop
1. If a reset clears the error and the boiler runs normally, monitor the system closely for recurrence. If the fault returns within a short time, stop and call an engineer. 2. If pressure topping up and radiator bleeding do not restore circulation and the boiler still shows the same error, suspect a stuck pump, blocked pipework, closed valve or failure of a flow/pressure sensor. 3. If the display pattern corresponds to a PCB/control fault (flash on flash) or you see additional PCB-related error codes, do not attempt internal electrical repairs—this requires a qualified engineer. 4. If you cannot identify low pressure, air, or condensate freezing as the cause, or the boiler has signs of overheating/dry firing (e.g., hot boiler casing, burn smell, or unusual noises), switch the boiler off and call a Gas Safe engineer immediately.
When to call a professional
1. Call a Gas Safe registered engineer if: the fault persists after basic checks and a single reset; the boiler indicates a PCB or internal control fault; the pump appears seized or there is suspected blockage; there are electrical faults or repeated lockouts; you smell gas or see leaks. 2. Do not attempt PCB replacement, internal wiring work, gas valve checks or flame sensing/ignition repairs yourself. These are gas/electrical safety critical tasks and must be undertaken by a qualified engineer.
Information to give the engineer
1. The exact display pattern and any error codes shown, and whether the code is intermittent or permanent. 2. What you have already tried (reset, pressure top-up, radiator bleeding, condensate thawing). 3. Any symptoms you noticed (no hot water, cold radiators, gurgling, unusual noises, leaks, smell of gas).
Summary: perform safe, simple checks first (pressure, bleed, reset). If the boiler continues to show ON FLASH FLASH, or if you suspect PCB, pump failure, dry fire or any gas/electrical issue, arrange a Gas Safe engineer to diagnose and repair the boiler.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Baxi Solo.