Alpha boiler fault codes: full list and what to do

Key facts

The short version before you scroll:

• Alpha shows faults as E-numbers (E10, E01, E37 and so on) across the E-Tec, InTec, NX and Evoke ranges.
• E10 means low water pressure — usually a simple top-up you can do yourself.
• E37 has two meanings: low supply voltage on older ranges, or a pressure-sensor fault on the NX range — both need an engineer.
• Anything involving gas, flame, the fan, flow or the PCB is a job for a Gas Safe registered engineer.
• Smart Plan is a service plan, not insurance. On the boiler and central heating module, parts and labour are included up to £500 a year if your boiler is under 7 years old, or up to £200 a year if it's over 7 years. These are annual limits shared across every fault in the 12-month period, so a larger repair can cost more than the cover pays and you'd cover the difference.
• Cover-plan call-outs run Mon–Fri 08:00–18:00 (with genuine emergencies, as defined in the plan terms, the only exception), and a £95 call-out fee applies in the cases set out in the plan terms. Once you use a service, a 12-month agreement period begins.

Alpha boiler fault codes at a glance

Alpha boiler fault codes (shown as E-numbers like E10, E01 and E37) each point to a specific fault. E10 is low water pressure and is usually a simple top-up, while codes involving gas, flame, the fan or the PCB need a Gas Safe registered engineer. Use the table below to identify your code and the safe next step.

Alpha is a common choice in new-builds and rented homes, so these codes turn up across the InTec, E-Tec, CD18R, Evoke and NX ranges. Most read the same from model to model, but a few differ — E37 is the big one to watch, which we cover in full below.

Find your code in the table first, then read the matching section underneath for what's safe to try and when to book an engineer.

Common Alpha fault codes

Alpha E10: low water pressure (the most common code)

E10 is the most common Alpha code, and it's the friendliest to see. It means your system pressure has dropped below roughly 0.6 bar, so the boiler stops firing to protect itself. In most cases you can fix it yourself by topping the system back up to around 1.0–1.5 bar using the filling loop.

Low pressure is often caused by recently bled radiators, a small drop over time, or a minor leak. Before you top up, have a quick look around the boiler and under radiator valves for damp patches — if you find water, book an engineer rather than repressurising.

How to repressurise an Alpha boiler for E10

Work through these in order. If you're not comfortable at any point, stop and book an engineer.

• Step 1 — Turn the boiler off and let it cool. Find the filling loop, usually a silver braided hose beneath the boiler with a valve at each end.
• Step 2 — Open both valves slowly. You'll hear water moving into the system and the pressure gauge will start to rise.
• Step 3 — Watch the gauge and close both valves once it reads between 1.0 and 1.5 bar. Don't overfill.
• Step 4 — Turn the boiler back on and reset it once if the code is still showing.
• Step 5 — If the pressure keeps dropping over the next few days, you likely have a leak or a failed expansion vessel. That needs a Gas Safe registered engineer.

Alpha E37: why it means two different things

E37 is the one Alpha code you can't take at face value, because it means two different things depending on your model. Check which range you have before you do anything, because the diagnosis is completely different.

On older Alpha ranges: low supply voltage

On older Alpha boilers, E37 signals low supply voltage — the boiler isn't getting enough mains voltage to run. This is often a fuse or fused-spur issue, or a fault with the PCB transformer. You can safely check that the fused spur feeding the boiler is switched on and that the fuse hasn't blown, but the transformer itself is an engineer's job.

On the Alpha NX range: a pressure-sensor fault

On the NX range, the same E37 code means the boiler can't read the correct system pressure — typically a dirty, faulty or disconnected pressure sensor. This isn't a DIY fix. A Gas Safe registered engineer will test and, if needed, replace the sensor.

Whichever version applies to you, once you've ruled out the fused spur, book an engineer to diagnose the transformer or the sensor. Your model's manual will confirm which meaning applies.

Other common Alpha codes (E02, E03, E16, E20, E25, E27, E47)

The rest of the common Alpha codes almost all point at gas, flame, the fan or the flow of water through the system — which means they're engineer territory, not DIY. Here's the one-line version of each so you know what you're looking at.

E02 is an overheat boiler lockout, usually a pump or flow problem. E03 is a high flue thermostat temperature, often a flue blockage or overheat. E16 is a fan fault. E20 is a flame-sensing fault and a deliberate safety shutdown. E25 is an overheat lockout caused by air or restricted circulation. E27 is insufficient primary flow, pointing to a blockage or pump fault. E47 is reduced burner output triggered by high flue temperatures.

For all of these, the safe next step is the same: book a Gas Safe registered engineer. You can reset the boiler once to see if it was a one-off, but if the code comes straight back, don't keep resetting it.

Which Alpha faults can I fix myself?

The honest answer is: not many, and that's a good thing. Low-pressure codes like E10 you can usually sort with a top-up. For a voltage-related E37 you can check the fused spur. And you can reset the boiler once to clear a one-off glitch. Everything else — anything touching gas, flame, the fan, the flow or the PCB — needs a Gas Safe registered engineer. It is illegal, and dangerous, to work on gas appliances unless you're Gas Safe registered.

Safe checks before you call anyone out

These are the only things worth trying yourself. If a code involves gas, flame or the fan, skip straight to an engineer.

• Check the pressure gauge — it should sit around 1.0 to 1.5 bar when the system is cold.
• Top up via the filling loop if pressure's low (E10), then close both valves fully.
• For a voltage-related E37, check the fused spur feeding the boiler is switched on and the fuse hasn't blown.
• Make sure the boiler has power and the programmer is set to bring the heating on.
• Reset the boiler once — just once. Don't keep resetting if the same code returns.
• If you smell gas, stop. Call the National Gas Emergency Service on 0800 111 999, open windows, turn off the gas at the meter, don't touch electrical switches, and leave the property.

When to call a Gas Safe engineer

If you've done the safe checks and the code won't clear — or the code involves gas, flame, the fan or the flow — it's time for a Gas Safe registered engineer. Repeatedly resetting a boiler that keeps locking out can hide a developing fault, so it's better to get it looked at.

Smart Plan is a service plan, not insurance. Spotting your Alpha code early lets you weigh up a one-off repair against setting up an ongoing boiler and central heating module so a Gas Safe registered engineer is arranged next time a code appears.

If you choose the module, parts and labour are included up to your cover limit — up to £500 a year if your boiler is under 7 years old, or up to £200 a year if it's over 7 years. Those are annual limits shared across every fault in the 12-month period, so if a repair costs more than your limit you pay the difference. Cover is modular, so you only pick what you want. A £95 call-out fee applies in the cases set out in the plan terms, and cover-plan call-outs run Mon–Fri 08:00–18:00, with genuine emergencies (as defined in the plan terms) the only exception. Once you use a service, a 12-month agreement period begins; a 14-day cooling-off period applies and voids once a service is used.

Alpha boilers are common in rented homes, so if you're a landlord managing gas safety and repairs across a property, our landlord cover is worth a look. Smart Plan is provided by UK Boiler Company Ltd, trading since 2014, and the company says it has looked after over 15,000 customers. If an Alpha code won't clear after the safe checks, you can book a one-off repair or set up a plan — call us on 0333 772 6247.

Alpha fault code FAQs

Alpha code still showing?

Book a one-off repair, or set up an ongoing Smart Plan boiler and central heating module. It's a service plan, not insurance: parts and labour up to £500 a year (boiler under 7 years) or £200 a year (over 7 years) — an annual limit shared across all faults, so a bigger repair can cost more than the cover pays. A £95 call-out fee applies in the cases set out in the plan terms; call-outs are Mon–Fri 08:00–18:00 (genuine emergencies aside, as defined in the plan terms); and once you use a service a 12-month agreement period begins.

Helpful links

• Boiler fault codes hub
• Boiler pressure too low — how to fix it
• How to bleed a radiator
• Boiler not working? Book a repair
• Boiler & central heating cover
• Landlord cover
• Check an engineer on the Gas Safe Register
• Alpha Heating Innovation — official site