Alpha Red A and B Illuminated Fault Code: Causes, Fixes & Repair Costs
What does the Alpha Red A and B Illuminated fault code mean?
When both the A and B indicator lights on your Alpha boiler are illuminated continuously in solid red, the boiler is telling you that the burner has failed to light. The boiler has locked out as a safety measure and will not attempt to fire again until the fault is cleared. It is worth noting that the A and B lights can display several different patterns, each with a different meaning: both lights flashing alternately points to low system pressure; both flashing simultaneously suggests a blocked flue or fan fault; A flashing with B illuminated indicates a pump or restricted flow issue. This page focuses specifically on both A and B lit continuously and solidly — the burner ignition lockout.
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
- Ignition electrode fault Common
The ignition electrode is responsible for producing the spark that lights the burner. If the electrode is worn, cracked, or its gap has drifted out of specification, the spark may be too weak or absent entirely, preventing the burner from lighting. This is one of the most frequent reasons for a repeated ignition lockout on Alpha boilers.
- Gas supply interruption Common
If the gas supply to the boiler is insufficient or has been interrupted — for example, if a gas meter valve is partially closed, a prepayment meter has run out of credit, or there is a wider supply fault in your area — the burner simply cannot light. Always check that other gas appliances in your home are working before assuming there is a boiler fault.
- Blocked or dirty burner Common
Over time, debris, dust, or corrosion can accumulate on the burner, restricting gas flow and preventing consistent ignition. A blocked burner often produces repeated lockouts and is unlikely to clear with a reset alone. This typically requires a Gas Safe engineer to clean or replace the burner assembly.
- Faulty gas valve Sometimes
The gas valve controls the flow of gas to the burner. If it fails to open correctly — whether due to an electrical fault, a seized component, or wear — no gas reaches the burner and ignition cannot occur. A faulty gas valve must be diagnosed and replaced by a Gas Safe registered engineer.
- Frozen or blocked condensate pipe Sometimes
In cold weather, the condensate pipe that carries waste water away from the boiler can freeze solid. When blocked, the boiler locks out as a safety precaution. This is more likely in winter if the condensate pipe runs outside or through an unheated space.
- Faulty flame sensor or detection circuit Sometimes
Even if the burner lights successfully, the boiler may lock out if the flame sensor fails to detect the flame and confirm ignition to the control board. A dirty or failing ionisation probe is a common cause of repeated lockouts where the burner briefly fires then cuts out.
- PCB or wiring fault Rare
A fault on the printed circuit board or a loose wiring connection to the ignition components can prevent the ignition sequence from completing. PCB faults are less common but should be considered if all other components have been checked and found to be in order.
How to fix it
- Check that your gas supply is working DIY safe
Turn on another gas appliance in your home — a gas hob or gas fire, for example. If those work normally, the supply to your property is fine. If nothing gas-powered works, contact your gas supplier or call the National Gas Emergency Service on 0800 111 999 immediately.
- Reset the boiler DIY safe
On Alpha boilers, turn the selector switch to the reset position (marked as position 3 on many models) and hold it there. The boiler will pause for approximately 25–30 seconds before attempting to restart the ignition sequence. You can attempt a reset two or three times, but do not reset repeatedly if the boiler keeps locking out — this can mask a more serious fault.
- Check the system pressure DIY safe
Look at the pressure gauge on the boiler. It should read between 1 and 1.5 bar when the system is cold. If it is below 1 bar, the boiler may not fire. Use the filling loop — usually a braided metal hose with one or two valves beneath the boiler — to top up the pressure. Open the valve(s) slowly, watch the gauge rise to around 1.2–1.5 bar, then close the valve(s) firmly. Note: if the A and B lights are flashing alternately rather than both lit solidly, low pressure is the likely culprit.
- Check for a frozen condensate pipe in cold weather DIY safe
If temperatures have dropped below freezing and the boiler's condensate pipe runs outside or through an unheated space, it may have frozen. You can thaw it by pouring warm (not boiling) water over the external section of the pipe, starting at the outlet end and working back towards the boiler. Once cleared, reset the boiler as described above.
- Call a Gas Safe registered engineer Gas Safe engineer
If the boiler continues to lock out after a reset, or if you cannot identify a simple cause such as low pressure or a frozen condensate pipe, the fault requires professional diagnosis. A Gas Safe engineer will inspect the ignition electrode, burner, gas valve, flame sensor, and flue to identify and resolve the root cause. Do not attempt to open the boiler casing or work on any gas components yourself — this is illegal without Gas Safe registration and is extremely dangerous.
Parts you may need
- Ignition electrode · from £35
- Flame sensor / ionisation probe · from £30
- Gas valve · from £120
- Burner assembly · from £80
- PCB (printed circuit board) · from £180
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 £120–£350, depending on the underlying cause.
Frequently asked questions
Can I just keep resetting my Alpha boiler when the red A and B lights come on?
You can try a reset two or three times, but repeatedly resetting a boiler that keeps locking out is not a solution and can sometimes make diagnosis harder. If the boiler relights and runs normally after a single reset, monitor it closely. If the fault returns within a short time, or if the boiler fails to light at all after resetting, you need a Gas Safe engineer to investigate the underlying cause.
What is the difference between the A and B lights flashing and being lit solidly?
The pattern of the A and B indicator lights matters. Both lit continuously and solidly means the burner has failed to light — which is what this page covers. Both flashing alternately usually indicates low system pressure, which you can often resolve yourself by topping up via the filling loop. Both flashing simultaneously points to a flue blockage or fan issue. A flashing with B solid suggests a pump or flow problem. Always note the exact pattern before calling an engineer, as it helps with diagnosis.
How much does it typically cost to fix this fault on an Alpha boiler in the UK?
For the most common causes — ignition electrode replacement, flame sensor, or a burner clean — most people pay between £120 and £350 including parts and labour. A gas valve replacement tends to sit toward the higher end of that range. More complex repairs such as a PCB replacement can cost more, and a heat exchanger failure — which is rare but possible — can run to £400–£600, at which point it is worth comparing the repair cost against a new boiler installation, particularly if your boiler is more than ten years old.
Is it safe to use my boiler while it is showing this fault?
No. When the A and B lights are illuminated solidly, the boiler has locked out and is not operating — so there is no immediate danger from the boiler running unsafely. However, you should not attempt to force the boiler to run by repeated resets if it is consistently failing to light. Leave diagnosis and repair to a Gas Safe registered engineer.