Alpha All Lights Flashing Fault Code: Causes, Fixes & Repair Costs
What does the Alpha All Lights Flashing fault code mean?
On Alpha boilers, a flashing-lights pattern is the primary way the controls communicate a fault rather than displaying a numeric code. There are three distinct flashing patterns you might see, each pointing to a different problem. First, if the A and B indicator lights flash simultaneously (both on and off at the same time), the boiler has entered a safety lockout — typically following a failed ignition attempt or an overheating event. Second, if the A and B lights flash alternately (one on while the other is off, taking turns), this specifically indicates that system pressure has dropped too low, usually below 1 bar. Third, if only the red A light flashes on its own (sometimes accompanied by a 2, 25, or 0A3 error code on compatible models), this points to an overheat thermostat fault. Identifying which pattern you are seeing is the essential first step before attempting any fix.
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 Common
The most common cause of the alternating A/B flash. If the pressure gauge reads below 1 bar or sits outside the green zone, there is not enough water in the sealed system to operate safely. Pressure can drop gradually through minor leaks, recently bled radiators, or normal system contraction over time.
- Ignition failure Common
When the boiler attempts to fire but the burner fails to light — or the flame is not detected — the controls lock out and both lights flash simultaneously. This can happen due to a dirty or worn ignition electrode, a faulty flame sensor, or a gas supply interruption.
- Overheat thermostat activation Common
If the water temperature inside the boiler exceeds safe limits, the overheat thermostat trips as a safety measure, causing the A light to flash red (sometimes alongside a 2, 25, or 0A3 code). This can be a one-off event or a symptom of a deeper problem such as a blocked heat exchanger or poor circulation.
- Faulty or seized circulation pump Sometimes
A pump that is not moving water efficiently — whether due to mechanical failure, seized bearings, or air trapped in the pump head — will cause heat to build up and trigger a lockout. Symptoms often include gurgling sounds from the boiler or radiators that remain cold.
- Fan or flue obstruction Sometimes
The fan must run before ignition to purge the flue of any residual gases. If the fan is faulty or the flue is blocked by debris or a bird's nest, the boiler will not proceed to light the burner and will lock out with both lights flashing simultaneously.
- Blocked or faulty burner Rare
A burner clogged with combustion deposits or a burner component that has failed will prevent reliable ignition or a stable flame, leading to repeated lockouts. This is less common but more likely on older boilers that have not been serviced regularly.
How to fix it
- Identify which flashing pattern you are seeing DIY safe
Before doing anything else, look carefully at the A and B indicator lights. Are both flashing at the same time (simultaneous — points to lockout), are they taking turns (alternating — points to low pressure), or is only the A light flashing (points to overheat thermostat)? The correct next step depends entirely on which pattern is showing.
- Check your gas supply is working DIY safe
If you are seeing a simultaneous lockout, quickly confirm that other gas appliances in your home — a hob ring, for example — are working normally. If there is no gas supply at all, contact your gas supplier before resetting the boiler.
- Top up system pressure if the lights are flashing alternately DIY safe
Locate the filling loop, which is typically a braided hose with one or two valves found beneath the boiler. Slowly open the valve (or valves) a quarter of a turn to allow mains water into the system. Watch the pressure gauge rise and stop when it reaches between 1 and 1.5 bar, firmly within the green zone. Close the valve(s) fully. The alternating flash should clear once pressure is restored.
- Bleed radiators if you have recently noticed cold spots or lost pressure DIY safe
Air trapped in the system can cause pressure loss and poor circulation. Use a radiator bleed key on the bleed valve at the top of each radiator until water (not air) escapes steadily, then retighten. Check the system pressure again afterwards and top up if needed.
- Reset the boiler DIY safe
On most Alpha models, rotate the front selector dial to the RESET position, hold it there for around 30 seconds, then return it to your normal heating or hot water setting. The boiler should attempt to re-ignite. If it locks out again straight away, do not keep resetting — repeated lockouts on the same fault indicate an underlying problem that needs engineer diagnosis. Limit resets to two or three attempts.
- Call a Gas Safe registered engineer if the fault persists or recurs Gas Safe engineer
If the boiler locks out again after resetting, if the A light is flashing alone (overheat thermostat fault), if you suspect an ignition, fan, pump, or burner issue, or if you are at all unsure, you must contact a Gas Safe registered engineer. It is a legal requirement that only Gas Safe registered engineers work on the internal gas components of a boiler. Do not remove the boiler casing or attempt internal repairs yourself.
Parts you may need
- Circulation pump · from £85
- Ignition electrode · from £25
- Flame sensing electrode · from £20
- Overheat thermostat (high-limit stat) · from £18
- Flue fan (draught inducer) · from £95
- Filling loop assembly · 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 £120–£380, depending on the underlying cause.
Frequently asked questions
Why are all the lights flashing on my Alpha boiler?
Alpha boilers use indicator lights rather than numeric codes to show faults, so the specific pattern matters. Both lights flashing simultaneously signals a safety lockout after a failed ignition or overheating event. Both lights flashing alternately means system pressure is too low — check the pressure gauge and top up via the filling loop if it is below 1 bar. If only the red A light is flashing, the overheat thermostat has tripped. Each pattern needs a different response.
Can I fix the flashing lights on my Alpha boiler myself?
Some causes are safe for a homeowner to address. If the A and B lights are alternating, you can top up the system pressure using the filling loop yourself. You can also bleed radiators and perform a boiler reset. However, anything involving the internal components — the ignition system, fan, pump, burner, or thermostat — must only be carried out by a Gas Safe registered engineer. Removing the boiler casing without being Gas Safe registered is illegal.
How much will it cost to repair my Alpha boiler if the lights are flashing?
For the most common repairs — a pump replacement, a new ignition or flame electrode, or an overheat thermostat — most people pay between £120 and £380 including parts, labour, and VAT. A straightforward pressure top-up by an engineer at the lower end of a callout charge, or a pump swap at the higher end, typically falls within this range. More involved work such as a heat exchanger replacement or a PCB fault can cost significantly more (£400–£700 or beyond), but these are less common outcomes for this fault pattern.
How do I reset an Alpha boiler?
Rotate the selector dial on the front of the boiler to the RESET position and hold it there for approximately 30 seconds, then turn the dial back to your normal operating setting (usually the flame or central heating symbol). The boiler will then attempt to fire. If it goes straight back into lockout, do not keep resetting — two or three attempts is the safe limit. Repeated lockouts mean a Gas Safe engineer needs to investigate the root cause.
My Alpha boiler keeps losing pressure and the lights flash alternately — is there a leak?
If you find yourself topping up the pressure more than once or twice a year, it is worth having a Gas Safe engineer check for a slow leak. Common sources include a leaking radiator valve, a weeping pressure relief valve, or a small drip from a pipe joint. A one-off pressure drop after bleeding radiators is normal, but recurring pressure loss should always be investigated rather than simply topped up repeatedly.