Overview
ON ON FLASH on a Baxi Solo normally indicates a printed circuit board (PCB) error or communication fault on that control board. The PCB is the boiler’s main electronic controller; it manages ignition, pumps, fans and reads sensors. When the PCB reports an error it means the controller has detected an internal fault or a mismatch/communication problem with connected components. Severity ranges from temporary/soft faults that clear after a reset to a hard PCB failure that will prevent the boiler from operating. Because the PCB controls safety-critical functions (gas ignition, flame monitoring, ventilation), this is not a simple cosmetic problem and usually requires a Gas Safe registered engineer or qualified electrical/boiler technician. Some basic non-invasive checks and a reset can be done by a homeowner, but repairing or replacing the PCB or doing any internal electrical or gas work should be left to a professional.
Possible Cause: Printed circuit board (PCB) error
Troubleshooting Steps
Safety first
- If you smell gas, evacuate premises immediately and call the national gas emergency number — do not attempt any checks.
- Before touching any controls, switch the boiler off at the main switch or isolator. If you plan to make any electrical checks, isolate the electrical supply to the boiler at the fused spur or consumer unit.
- Do not open the gas valve, remove casing panels to access live electrical parts, or attempt PCB repairs unless you are a qualified engineer — internal work risks electric shock, gas leakage and invalidating warranties.
Initial checks a homeowner can safely do
1) Note and record the exact fault display sequence (ON ON FLASH) and any other lights or error codes shown. Also record boiler model and serial number. These details help the engineer diagnose the fault.
2) Try a simple reset: follow the boiler’s reset procedure (press and hold the reset button or turn the selector to R for 5 seconds depending on your unit). Wait a few minutes to see if the boiler restarts normally. Do not repeatedly attempt resets more than 2–3 times.
3) Check the mains electrical supply to the boiler: confirm other household lights/appliances are working, and check the boiler’s fused spur or isolator isn’t tripped. If your household voltage is very low (brownouts) the boiler can show electronic faults; if you suspect low mains voltage call an electrician or your supply company.
4) Check for obvious external issues: low heating pressure on the gauge, recent power cuts, visible signs of water around the boiler, or a burnt smell near the unit. Take photos of any visible damage and of the fault lights to show the engineer.
Further diagnostic information for the engineer (what you can prepare)
- Make a note of any recent events before the fault (power cut, lightning strike, plumbing work, boiler banging, condensate pipe freeze, or water leak).
- Confirm whether other fault codes or flashing sequences have occurred previously and whether any other functions (hot water, heating) work intermittently.
- If comfortable and safe, take clear photos of the PCB area (after switching off and isolating power) showing connectors and any visible corrosion or water ingress — only do this if the outer panel is designed for user access and no internal live-work is required. If unsure, skip and wait for the engineer.
Diagnostic and likely fix steps a professional will take
- The engineer will run mains and PCB voltages, check the boiler’s fault history and communication between PCB and sensors/actuators (fan, pump, flame sensor, thermistors). They will inspect for water damage, burned components, loose connectors, blown PCB fuses or bad solder joints.
- They will confirm whether the fault is a temporary parameter/firmware glitch that a controlled reset or firmware reconfiguration clears, or a hardware failure requiring PCB repair or replacement. If connectors or wiring harness faults are found, they will be repaired or replaced.
- If the PCB is replaced, the engineer will re-check all safety circuits, gas function, and run the boiler through fault checks and commissioning procedures.
When to call a professional
- If the reset does not clear the ON ON FLASH, if the fault returns, or if you notice water on or near the boiler, a burnt smell, frequent lockouts, or other sensor/fan faults, contact a Gas Safe registered engineer. PCB faults affect safety-critical controls and require specialist diagnosis and safe replacement.
Notes and practical tips
- Do not repeatedly reset the boiler multiple times — frequent resets can mask intermittent faults and may worsen damage.
- When you call the engineer, provide the boiler model, serial number, exact fault sequence (ON ON FLASH), any photos, and notes of events that preceded the fault — this speeds diagnosis.
- Warranty or manufacturer support may cover PCB replacement if the unit is still under warranty; let the engineer check warranty status before unauthorised repairs.
(Do not attempt to repair or replace the PCB yourself unless you are a suitably qualified technician; this guidance is for safe initial checks only.)
Helpful Resources
Boiler Manual
Download the official PDF manual for the Baxi Solo.