Alpha 0A43 Fault Code: Causes, Fixes & Repair Costs
What does the Alpha 0A43 fault code mean?
The 0A43 fault (also displayed as E43 on many Alpha models — both codes refer to exactly the same fault) means your boiler has lost its flame signal during operation and shut down as a safety precaution. The boiler's control board either detected that the burner flame went out unexpectedly, or it lost communication with the flame-sensing circuit entirely. Because the flame sensor and the PCB are closely linked, a fault in either component — or in the wiring between them — can trigger this same code. The boiler will not restart until the underlying cause is resolved and the fault is cleared.
General guidance only — not a substitute for professional advice, and not certified by a Gas Safe engineer. Always have your specific appliance assessed by a qualified professional. Any gas work must be carried out by a Gas Safe registered engineer. If you smell gas or suspect carbon monoxide, leave the property and call the National Gas Emergency line on 0800 111 999.
Common causes
- Gas supply interruption Common
If there is no gas reaching the burner — because the meter valve is closed, a pre-payment meter has run out, or a wider supply issue exists — the boiler will ignite briefly then lose flame immediately, triggering 0A43. Check that other gas appliances (cooker, hob) in the property are working normally.
- Frozen condensate pipe Common
In cold weather the plastic condensate pipe that carries waste water from the boiler to an outside drain can freeze solid. This blocks drainage and causes the boiler to lock out, often showing flame-related fault codes. The pipe is typically white or grey plastic, 21–32 mm in diameter, and runs to an external wall or drain.
- Low system pressure Common
If the boiler's pressure gauge reads below about 0.8 bar, there may be insufficient water in the system for the boiler to fire and sustain a flame correctly. The normal cold operating pressure for most Alpha boilers is 1.0–1.5 bar.
- Faulty or dirty flame sensor electrode Sometimes
The flame sensor (ionisation electrode) detects whether a flame is present. If it is coated with carbon deposits, cracked, or its lead is damaged, it will fail to send a reliable signal to the PCB — causing the boiler to shut down even when combustion is occurring normally.
- Flue gas recirculation or high wind Sometimes
If the flue terminal is poorly positioned or exposed to strong prevailing winds, combustion gases can be drawn back into the air intake. This disrupts the burner flame and causes repeated flame-loss events, leading to a 0A43 lockout.
- Faulty or incorrectly set gas valve Sometimes
A gas valve that is sticking, drifting out of calibration, or failing to open fully will result in insufficient gas pressure at the burner, causing the flame to extinguish shortly after ignition. Gas valve diagnosis and adjustment must only be carried out by a Gas Safe registered engineer.
- PCB or wiring fault Rare
The PCB controls the entire flame-sensing circuit. Damaged wiring, corroded connectors, or a failing PCB can cause the control board to lose communication with the flame sensor, generating a 0A43 code even when the gas supply and sensor are physically sound.
How to fix it
- Check that your gas supply is on and working DIY safe
Turn on a gas hob or check another gas appliance in the property. If nothing gas-powered works, contact your gas supplier — the issue is outside your boiler. If the meter has a pre-payment facility, make sure it has adequate credit.
- Check the boiler pressure gauge DIY safe
Look at the pressure gauge on the front of the boiler. It should read between 1.0 and 1.5 bar when the heating is cold. If it is below about 0.8 bar, top it up using the filling loop — a braided or plastic hose connecting the two valves beneath the boiler. Open both valves slowly, watch the gauge rise to around 1.2 bar, then close both valves. Never exceed 2.0 bar.
- Inspect the condensate pipe for freezing (cold weather only) DIY safe
Trace the white or grey plastic pipe from the bottom of the boiler to where it exits the property. If the pipe feels solid or you can see ice, pour warm (not boiling) water along the frozen section. A hot water bottle held against the pipe also works well. Once thawed, reset the boiler.
- Reset the boiler DIY safe
Press and hold the reset button (usually marked with a flame symbol or 'Reset') for around three seconds until the boiler attempts to relight. Allow a full ignition cycle. If the fault clears and the boiler runs normally, monitor it over the next hour. If the 0A43 code returns promptly, do not reset more than two or three times in total — repeated unsuccessful resets will not fix an underlying fault and can mask the problem.
- Check for obvious flue obstructions DIY safe
From outside, look at the flue terminal on the wall or roof. Make sure it is not blocked by a bird's nest, overgrown vegetation, or debris. Do not attempt to dismantle the flue — just visually confirm the outlet is clear.
- Call a Gas Safe registered engineer to diagnose and repair Gas Safe engineer
If the 0A43 fault persists after the checks above, the cause is likely a faulty flame sensor, damaged electrode lead, gas valve issue, or PCB fault. All of these require a Gas Safe registered engineer to test and repair safely. Ask the engineer to check the flame sensor and its wiring first, as this is often the most straightforward fix. Provide them with the full fault code — 0A43 or E43 — when booking.
Parts you may need
- Flame sensor / ionisation electrode · from £25
- Electrode lead / sensor wiring harness · from £20
- Gas valve · from £120
- PCB (printed circuit board) · from £280
The exact spare depends on your boiler's GC number (on the data badge). Check this against the part before buying.
Typical repair cost
Expect to pay roughly £100–£350, depending on the underlying cause.
Frequently asked questions
Is 0A43 the same as E43 on an Alpha boiler?
Yes, they are identical. Some Alpha display panels prefix fault codes with '0A', so the same fault appears as '0A43' on certain models and simply 'E43' on others. The underlying meaning, causes, and repair process are exactly the same whichever version your boiler shows.
Can I fix Alpha fault code 0A43 myself?
There are a few safe checks a homeowner can do: confirming the gas supply is on, topping up the boiler pressure to around 1.2 bar, thawing a frozen condensate pipe in winter, and resetting the boiler once or twice. If none of these resolve the fault, the repair requires a Gas Safe registered engineer. Work on the flame sensor wiring, gas valve, or PCB must never be attempted without Gas Safe registration — it is unsafe and can invalidate your warranty.
How much does it cost to repair an Alpha boiler showing fault code 0A43?
Most repairs for this fault fall in the range of £100–£350. A flame sensor or electrode replacement is towards the lower end, while a gas valve replacement typically costs around £180–£250 including labour. If the PCB turns out to be faulty, replacement parts and labour can reach £400–£650 — and on a boiler that is already eight to ten years old, it is worth comparing that cost against a new boiler installation before committing to the repair.
Why does my Alpha boiler keep showing 0A43 after I reset it?
If the fault returns within a short time of resetting, the boiler is detecting a genuine and persistent problem — most likely a faulty flame sensor, a gas supply issue, or a failing PCB. Repeatedly resetting will not fix the root cause. Limit resets to two or three attempts, then book a Gas Safe engineer to carry out a proper diagnosis rather than continuing to reset and risk masking a more serious fault.