Error ON ON ON

Safety precautions:

1. If you smell gas, evacuate the premises immediately, turn off the gas supply at the meter if you can safely do so, and call the gas emergency number. Do not operate electrical switches or the boiler.

2. Before opening the boiler casing turn off the electrical supply to the boiler at the isolator or the fuse box. If you are not confident with electrical isolation or working on appliances, stop and call a Gas Safe engineer.

3. Wear safety gloves and eye protection when inspecting inside the casing. Be aware there are hot surfaces and sharp edges.

Initial homeowner checks you can do (no tools or minimal tools):

1. Check the boiler display and any error log. Take a clear photo of the ON ON ON display and note whether the lockout light is steady or flashing.

2. Try a basic reset: use the boiler reset button or selector (hold for 5–10 seconds on models with a reset function). Wait and observe if the boiler attempts to restart and whether the error returns. Do not keep repeatedly resetting more than 2–3 times.

3. Check external controls: verify the room thermostat and programmer are calling for heat, and replace thermostat batteries if fitted. If you have a wired thermostat, temporarily bypass it (if your system allows and you know how) to test whether the boiler will fire without the thermostat in the circuit.

4. Check the fused spur or isolator switch and the consumer unit (fuse/trip) for the boiler circuit. Make sure the boiler is receiving mains power and other appliances on the same circuit are working.

5. Check system pressure on the boiler gauge and confirm it is within the normal range (typically 1.0–1.5 bar). Low pressure can cause other faults and may confuse diagnostics.

Specific diagnostic and limited fix steps (only proceed if competent with basic electrical checks):

1. Visual inspection: with the boiler powered off at the isolator and after allowing components to cool, carefully remove the outer casing and visually inspect the low-voltage wiring harness, connectors and the area around the PCB and PU module. Look for loose connectors, disconnected plugs, corroded pins, burn marks or damaged insulation.

2. Reseat connectors: if you see multi-pin plugs that look partially out or dusty, ensure the power is off and reseat them firmly. Do not force connectors; align and push until they click. Photograph the connections before you disturb them so the engineer can see the original layout if needed.

3. Check visible earths and cable entries: ensure cables are properly clamped and not chafing. A loose earth or damaged conductor can cause communication faults.

4. Basic power check (only if you know how to use a multimeter and are comfortable with live checks): isolate the boiler, remove casing, reconnect power, and measure the low-voltage supply points as per the manual. Typical checks are transformer secondary voltage or control-voltage terminals. If you are not familiar with live testing, do not attempt this — call an engineer. If measured voltages are absent or significantly below expected values, that confirms a supply or transformer issue.

5. If the fault appears tied to an external control (smart thermostat, wired thermostat), disconnect that control and try a direct call for heat from the boiler controls. If the error clears, the problem is likely with the external control or its wiring.

6. After any reconnections, restore the boiler casing, turn power back on and attempt a controlled restart. If the ON ON ON error clears and boiler operates normally for a sustained period, monitor over the next 24–48 hours for recurrence.

When to call a professional and what to tell them:

1. Call a Gas Safe registered heating engineer if the error persists after the basic checks and reset, if you found damaged wiring, burnt connectors or if you are unsure at any stage. Also call an engineer if you measured abnormal voltages or the boiler locks out repeatedly.

2. Give the engineer the boiler model (Baxi Solo), the exact error displayed (ON ON ON), the actions you have already taken (reset attempts, reseated connectors, photos) and whether external controls were tested or bypassed. This helps reduce diagnostic time.

3. Do not attempt mains wiring repairs, PCB replacement, transformer replacement or gas valve/combustion unit repairs yourself unless you are a qualified, registered engineer.

Final note:

Low-voltage wiring harness faults can be simple (loose plug or flat thermostat batteries) or serious (failing transformer, PCB or burnt harness). Because both electrical safety and gas safety are involved, aim to perform only the non-invasive checks listed above and arrange a Gas Safe registered engineer for any internal electrical repairs, component replacements or if the fault is not clearly resolved by basic troubleshooting.