Overview
ON OFF FLASH on a Baxi Solo commonly indicates an electrical supply fault that the boiler control electronics are detecting. The specific message you supplied — "possible live and neutral reversed" — means the boiler thinks the incoming mains conductors may be connected the wrong way round (polarity reversed) or there is a serious wiring/connection problem at the boiler power input or fused spur. The boiler monitors mains voltages and internal sensors; if it detects unusual voltages, loss of neutral, or incorrect wiring it will lock out or flash to protect the PCB and other components. This is a potentially serious fault. Incorrect mains wiring can damage the boiler electronics, create unsafe electrical conditions, and cause unpredictable behaviour. It is not a routine plumbing fault that follows simple topping-up or radiator bleeding. While you can carry out a few basic safety checks and a controlled reset, diagnosing and fixing reversed mains polarity or internal wiring faults requires qualified electrical work and a Gas Safe registered engineer to ensure the boiler is safe to return to service.
Possible Cause: Live and neutral reversed error
Troubleshooting Steps
Safety precautions:
1. If you smell gas, do not touch the boiler. Evacuate the property, call the gas emergency number and a Gas Safe engineer immediately.
2. If you are not a competent electrician, stop at the initial checks below and call a qualified electrician plus a Gas Safe engineer for the boiler. Do not attempt live work on the mains or inside the boiler without proper qualifications.
3. Isolate electrical power to the boiler at the fused spur or consumer unit before touching any wiring or the boiler casing. Confirm power is off with a suitable tester.
4. Do not repeatedly reset the boiler if the same error returns. Repeated resets can damage the PCB.
Initial checks a homeowner can do safely:
1. Note exactly how the boiler displays the fault and take a photo of the display. Try a single controlled reset: press and hold the reset/selector for 5–10 seconds and allow the boiler to attempt restart. Only do this once after checking the steps below.
2. Check other electrical appliances and lights in the house. If multiple devices are behaving oddly or RCDs/MCBs have tripped, this points to a house electrical issue, not just the boiler.
3. Check the fused spur for the boiler (if accessible) for a blown fuse. Replace the fuse only when you are certain the cause is not a wiring fault.
4. Check the consumer unit for tripped breakers or RCDs and reset them if appropriate. If breakers trip again, stop and call an electrician.
Specific diagnostic steps (only if you are confident and competent with electrical testing):
1. Isolate the boiler supply at the fused spur or consumer unit and ensure the boiler is powered down.
2. With power off, remove the boiler cover only if you are competent to do so and it is safe. Inspect the incoming mains terminal block for loose or burnt connections, signs of overheating, or incorrect wiring routes. Do not proceed if you are unsure.
3. Restore power and, using a properly rated voltage tester or multimeter, check polarity at the boiler supply terminals: L to N should read around 230V AC, L to earth about 230V, N to earth close to 0V. If readings are inconsistent or reversed, switch power off and call a qualified electrician. If you are not trained to use a meter, do not attempt this.
4. If you find a loose connector and you are competent to work electrically, isolate power, tighten the terminal screws, ensure correct conductor placement (L to L, N to N, E to earth), then restore power and test. If any signs of damage exist (burn marks, melted insulation), do not reconnect and call an electrician and a Gas Safe engineer.
After any corrective action:
1. Restore power and perform a controlled reset of the boiler once. Watch for the fault to reappear. If the boiler fires and operates normally, monitor for intermittent issues.
2. If the fault returns, stop attempts to reset and call a Gas Safe registered engineer and an electrician. The problem may be intermittent supply faults, a failed PCB, or other internal damage that needs professional replacement.
When to call a professional:
1. If you are not confident with any of the electrical checks above.
2. If the error persists after a single controlled reset.
3. If you detect reversed polarity, loss of neutral, loose or damaged wiring, burnt terminals, or repeated tripping of breakers.
4. If you smell gas, see damage or burning, or the boiler behaves unpredictably.
Who to call and why:
1. Call a qualified electrician to diagnose and correct any mains wiring, polarity, or supply faults. They will have the tools to test and correct polarity and repair electrical faults safely.
2. Call a Gas Safe registered boiler engineer to examine the boiler after the electrical supply is confirmed correct. If the supply fault has caused PCB or internal damage, the Gas Safe engineer can replace parts and confirm safe operation.
Final note: This fault deals with the mains supply and electrical safety. Do not attempt internal mains repairs unless you are a qualified electrician. Contact a Gas Safe engineer for any work that involves the boiler's gas or sealed parts.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Baxi Solo.