Overview
The ON FLASH ON display on a Baxi Solo is a fault pattern that frequently points to an electrical/sensor problem rather than a simple user-control issue. On many Baxi models similar flash patterns are associated with PCB (printed circuit board) faults or with NTC thermistor sensor failures (for example the central heating flow sensor or hot water sensor). A thermistor (NTC) measures water temperature and reports that to the PCB; if the reading is out of range or the sensor/wiring is open or shorted the boiler will register a fault. Severity: this is a moderate-to-high severity fault because the boiler will often lock out and stop providing heating and/or hot water until the cause is found and cleared. It is not normally an immediate gas-safety emergency by itself, but because the boiler controls combustion and safety interlocks, internal electrical or PCB faults should be diagnosed and repaired by a qualified Gas Safe registered engineer. A simple reset may clear a transient fault, but recurring ON FLASH ON means professional diagnosis is needed.
Possible Cause: Thermistor error
Troubleshooting Steps
Safety precautions:
1. If you smell gas, leave the property immediately and call your gas emergency number — do not attempt any checks.
2. If you are unsure or uncomfortable at any point, stop and call a Gas Safe registered engineer. Do not open the sealed boiler casing or attempt internal electrical repairs.
3. Isolate the boiler electrically at the fused spur or consumer unit before doing any work beyond basic external checks.
Initial homeowner checks (non-invasive):
1. Record exactly what you see: the full display pattern and whether the boiler has locked out or is trying to fire. Note whether hot water, central heating or both are affected.
2. Try a controlled reset once: use the boiler reset button or selector (consult your boiler’s front panel/handbook). If your model has an ON/OFF/SELECTOR switch, move to R (Reset) and hold for 5 seconds. If it has a reset button hold until the display clears. Wait for the boiler to attempt to restart.
3. Power-cycle: turn off the boiler at the fused spur, wait 60 seconds, then restore power and attempt restart.
4. Check simple system items: confirm the heating programmer/thermostat is calling for heat, check room stat batteries if battery powered, and check system pressure on the gauge (ideally around 1–1.5 bar when cold). If pressure is very low refill via the filling loop following your boiler manual.
5. Inspect externally visible items: check the condensate pipe for frost/ice if it’s cold outside (thaw with warm water, not boiling), look for obvious flue terminal obstructions, and ensure the external sensor/pipework (if fitted) isn’t damaged.
6. If the fault clears after reset but returns, note the circumstances (e.g. happens when calling for hot water, after long running, after a power cut) and stop further DIY.
If the fault persists or if you see signs of overheating, persistent lockout, abnormal noises or repeated fault codes:
1. Do not attempt to replace sensors or PCB yourself. These are live and gas-containing components.
2. Contact a Gas Safe registered engineer and tell them: boiler make/model (Baxi Solo), exact display text "ON FLASH ON", what you have tried (reset, power cycle, pressure check, condensate thaw) and any other error lights/codes.
What a qualified engineer will do (diagnostic outline):
1. Read the boiler fault memory and service menu to confirm the exact fault code mapping for your Solo model and any related logged faults.
2. Visually inspect wiring and connectors for the thermistor(s) and look for loose connectors, corrosion, or damaged cables.
3. Measure the NTC thermistor resistance at the sensor and at the PCB connector at different temperatures to confirm correct NTC behaviour (open circuit, short circuit or out-of-spec values indicate a faulty sensor or wiring).
4. Check the PCB for visible damage, burned tracks or loose connector pins. If a PCB fault is suspected the engineer will confirm whether the problem is the board itself or an external sensor causing a false fault.
5. Check related components that cause similar faults (fan, flow switch, pump, pressure sensor) if diagnostic data points to them.
6. Replace faulty component(s) only after proper diagnosis (thermistor, wiring, PCB) and carry out commissioning checks, run the boiler through multiple heat/demand cycles and confirm safe operation.
Final notes:
1. If a simple reset and basic checks do not permanently clear the ON FLASH ON fault, a Gas Safe registered engineer must be called. Do not leave the boiler in an unreliable state — this affects both comfort and safe operation.
2. When calling the engineer, provide model, serial, the exact fault display and what troubleshooting you have already performed to speed up diagnosis.
3. No part replacements or internal work should be carried out by anyone other than a qualified engineer. No cost estimates are provided here.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Baxi Solo.