Alpha Red A Yellow B Flashing Fault Code: Causes, Fixes & Repair Costs
What does the Alpha Red A Yellow B Flashing fault code mean?
On older Alpha boilers that use LED indicator lights instead of a digital display, the Red A and Yellow B lights flashing together is a dual-LED warning with two distinct meanings depending on how the lights are flashing. If the two lights flash simultaneously (at the same time), the boiler has locked out following a burner ignition failure — the boiler tried to light the burner, detected a problem, and shut down as a safety measure. If the two lights flash alternately (one then the other, back and forth), the fault points to low system pressure, typically below 1 bar on the pressure gauge. The quickest way to tell these apart is to glance at your pressure gauge: if it reads below 1 bar or outside the green zone, low pressure is almost certainly the cause. If pressure looks fine, you are dealing with a burner or ignition-related 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
- Low system pressure (alternating flash) Common
The most straightforward cause. If the Red A and Yellow B lights are flashing alternately, the boiler has detected that water pressure in the central heating system has dropped below the safe operating threshold of around 1 bar. This is often caused by a small water leak somewhere in the system, air bleeding from radiators, or natural pressure loss over time. Check the pressure gauge — if the needle is below 1 bar or outside the green section, this is almost certainly your fault.
- Burner ignition failure (simultaneous flash) Common
When the lights flash at the same time, the boiler has attempted to ignite the burner and failed, triggering a safety lockout. The ignition electrodes may be worn, cracked, or incorrectly positioned; the ignition leads may have a poor connection; or carbon deposits may have built up on the electrode tip. This is a common cause of burner lockout faults on Alpha boilers and requires a Gas Safe engineer to inspect and resolve.
- Blocked or faulty burner Sometimes
Debris, corrosion, or a manufacturing defect in the burner itself can prevent the gas from igniting correctly. Rather than a simple one-off misfire, a blocked or damaged burner will cause repeated lockouts and the dual LED warning. An engineer will need to remove, inspect, and either clean or replace the burner.
- Gas valve fault or gas supply interruption Sometimes
If no gas is reaching the burner — either because the gas valve has failed, the valve is stuck closed, or the gas supply to the property has been interrupted — the boiler will lock out. Check that other gas appliances in your home (hob, gas fire) are working. If they are not, contact your gas supplier. If they are, the gas valve inside the boiler is likely at fault and a Gas Safe engineer must investigate.
- Faulty burner fan Rare
The combustion fan must reach the correct speed before ignition is attempted. If the fan is failing, running too slowly, or not starting at all, the boiler's control board will prevent ignition and lock out. This typically requires fan replacement by a Gas Safe engineer.
How to fix it
- Check whether the lights are flashing simultaneously or alternately DIY safe
Stand in front of the boiler and watch the Red A and Yellow B LEDs closely for 10–15 seconds. If they flash at the same moment together, you have a burner lockout (see steps 3 onwards). If they alternate — one on while the other is off, switching back and forth — low system pressure is the likely cause and you should move straight to step 2.
- If flashing alternately: check and top up system pressure via the filling loop DIY safe
Locate the pressure gauge on your boiler — it is usually a small dial or display showing pressure in bar. If it reads below 1 bar, you need to repressurise. Find the filling loop, which is typically a short braided metal hose with one or two small tap or valve connections, usually underneath the boiler. Slowly open the tap(s) a quarter turn and listen for water entering the system. Watch the gauge rise. Once it reaches between 1 and 1.5 bar (the green zone), close the tap(s) fully. Do not overfill — above 2 bar is too high. The lights should stop flashing and the boiler should restart. If pressure drops again within a few days, there may be a leak in your system and an engineer should investigate.
- Attempt a boiler reset (for simultaneous flash / burner lockout) DIY safe
If pressure is normal and the lights are flashing simultaneously, try a reset. Depending on your Alpha model, this is done by pressing the dedicated reset button, or by turning the control dial back to position 3 and then returning it to your desired setting. Wait a few minutes after resetting before the boiler attempts to fire again. Do not reset the boiler more than two or three times — repeated resets without a successful start can mask underlying problems and, on a gas appliance, repeated failed ignition attempts should be investigated by an engineer.
- Check your gas supply DIY safe
Before calling an engineer, confirm that gas is reaching your property. Try a gas hob ring or another gas appliance. If nothing works, contact your gas supplier — there may be an interruption to supply in your area. If other appliances work fine, the fault lies within the boiler itself.
- Call a Gas Safe registered engineer to diagnose and repair the burner fault Gas Safe engineer
If the reset has not resolved the simultaneous-flash lockout, or if the boiler keeps locking out repeatedly, a Gas Safe registered engineer must inspect the boiler. They will check the ignition electrodes and leads, inspect and test the burner, examine the gas valve, and test the fan. Common repairs include cleaning or replacing the ignition electrode, replacing the burner, servicing or replacing the gas valve, or replacing the fan. Do not attempt to open the boiler casing or touch any internal components yourself — this is a legal requirement and a safety necessity.
Parts you may need
- Ignition electrode · from £25
- Ignition lead / HT lead · from £20
- Burner assembly · from £120
- Gas valve · from £150
- Combustion fan · from £130
- Filling loop (for pressure top-up) · from £15
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–£380, depending on the underlying cause.
Frequently asked questions
My Alpha boiler pressure keeps dropping — why does it lose pressure after I top it up?
If your boiler pressure drops repeatedly within days of topping up, there is almost certainly a water leak somewhere in your central heating system. This could be a weeping radiator valve, a pinhole leak on a pipe, a faulty pressure relief valve discharging water outside, or a failing expansion vessel. A Gas Safe engineer can pressure-test the system to find the source. Topping up the pressure yourself is a short-term fix, not a solution — persistent pressure loss needs investigating promptly to avoid damage to the boiler and heating system.
How do I tell whether my Alpha boiler's A and B lights are flashing together or alternately?
Watch the two LEDs carefully for about 15 seconds. If both lights come on and go off at the exact same moment — like two bulbs on the same switch — they are flashing simultaneously, which points to a burner lockout. If they take turns — Red A on while Yellow B is off, then Yellow B on while Red A is off, cycling back and forth — they are flashing alternately, which points to low system pressure. Checking your pressure gauge is the fastest confirmation: below 1 bar strongly suggests the alternating-flash low pressure fault.
Can I fix the Alpha Red A Yellow B fault myself, or do I need an engineer?
It depends on the cause. If the lights are flashing alternately and your pressure gauge reads below 1 bar, you can safely top up the system pressure yourself using the filling loop — no engineer needed. If the lights are flashing simultaneously (burner lockout), you can try resetting the boiler once or twice, but if it locks out again you will need a Gas Safe registered engineer. Any work on the gas valve, burner, ignition components, or fan must be carried out by a registered engineer. Removing the boiler casing yourself is not only unsafe, it is a legal requirement under UK gas safety regulations that this work is performed by a Gas Safe professional.
How much does it cost to fix an Alpha burner lockout fault in the UK?
For most homeowners, repairs for this fault come in between £100 and £380 including callout, labour, and parts. Cleaning and adjusting the ignition electrode or replacing ignition leads sits at the lower end, typically £100–£180. Replacing a burner or servicing a gas valve usually falls in the £200–£380 range. More significant components such as a PCB or heat exchanger can cost considerably more — sometimes £400–£700 — but these are less commonly the root cause of this specific fault code. If your Alpha boiler is already several years old and the repair quote is approaching £400 or above, it is worth getting a quote for a new boiler installation as a comparison.