Alpha E20 fault codewhat it means and how to fix it
Alpha call this a flame sensing fault and define it as false flame detection: the appliance can see a flame while the gas valve is shut. Switch it off and call an engineer.
Stop — this needs an engineer
Get a Gas Safe registered engineer to look at your Alpha E20
We do not pass enquiries to a repair firm, so there is no form here. Find a Gas Safe registered engineer through the Gas Safe Register, and ask to see their registration card when they arrive — it is the register, not the van, that tells you they are qualified.
What can you tell before anyone arrives?
The right answer depends on what you can see. Pick the one that matches.
Choose the option on the left that matches what your appliance is doing, and the answer for that case appears here.
You can smell gas anywhere in the property
Stop. Do not touch any switch, including to turn something off.
Leave the property, open a door or window on the way out, and ring 0800 111 999 from outside.
The code is showing and the boiler will not start
The appliance is doing what it is designed to do. It has refused to open the gas valve because it cannot prove the combustion chamber is empty.
Switch the boiler off at the fused spur and book a Gas Safe registered engineer. Say the code is a false flame fault when you book, because it changes what they bring.
It appears now and then and the boiler otherwise runs
Intermittency points at the wiring rather than the gas side, but it is not evidence that the gas side is sound and it is not a reason to wait.
Book it rather than watching it. Note when it appears, because a fault that follows wet weather is a strong pointer to the lead.
What E20 means
This code reads like the boiler failing to notice a flame and it is the opposite. Alpha give it two descriptions in their own error table, and the second is the one that matters — flame detected but gas valve is not open. The appliance runs its ionisation circuit before it starts, and the answer it expects is nothing, because nothing should be burning yet.
Getting a signal instead leaves it with two possibilities it has no way to tell apart: either the sensing circuit is lying to it, or gas is genuinely burning when it should not be. It refuses to open the gas valve either way, which is the whole point of the check.
The great majority of these turn out to be the first possibility — Alpha themselves list a short circuit on the sensing wires and a PCB fault alongside the electrode and its lead — and that is reassuring rather than conclusive. The appliance has flagged the one condition it cannot safely investigate on its own, and neither can you, because the check that separates a lying circuit from a leaking valve is a gas soundness test.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Short circuit on the sensing wires | Alpha name it explicitly. A chafed lead or moisture across a connector gives the board a conductive path that reads exactly like a flame, and the appliance has no way to distinguish it from one. | Common |
| Sensing electrode or its lead | Heavy carbon, a cracked ceramic or condensate tracking down the lead all create the same false path. The commonest of the benign explanations and the cheapest to put right. | Common |
| Control board fault | The stage that reads the ionisation current has failed in a way that reports a signal with no flame present. Diagnosed once the wiring and electrode have been eliminated, not before. | Occasional |
| A gas valve genuinely passing when closed | The least likely and the reason none of the others may be assumed. A valve seat holding a small amount of gas past means something really is alight in there, and only a soundness test settles it. | Rare |
Can you fix it yourself?
Stop — this needs an engineer
What it costs to fix
Typical repair cost
£110–£380
An electrode and lead replacement is £110 to £190 fitted. A wiring repair is often within a call-out of £80 to £150. A control board is £300 to £480, and a gas valve more.
Typical UK prices including parts and labour, updated September 2026. Your quote may differ.
Own this appliance and had this code? . Nothing is published until a person has read it.
Related Alpha fault codes
- E01Get it checkedThe appliance tried to light and could not confirm a flame, so it has locked out. In winter a frozen condensate pipe is the most likely reason.Related: ignition and gas supply
- E38Get it checkedThe burner lit correctly and then the flame signal was lost part-way through the cycle. That points somewhere different to a failure to light.Related: ignition and gas supply
- E10Get it checkedAlpha describe E10 as the primary pressure switch operating. Refilling to their green band usually clears it, but the switch itself is on their own list of causes.Related: same brand
- E02Get it checkedThe overheat thermostat has tripped. Almost always this means water is not moving through the appliance properly rather than that it is running too hot.Related: same brand
- E16Get it checkedThe control board cannot confirm the fan is running at the speed it asked for, so it will not let the burner light. This one needs an engineer.Related: same brand
- E60Get it checkedThe appliance cannot confirm the pump is moving water. Left alone it becomes an overheat fault, so it is worth acting on promptly.Related: same brand
Own an Alpha boiler? Save it and its codes come first next time.
Related guides
Common questions about Alpha E20
Is this dangerous, or is the boiler just confused?
Most likely confused, and treated as dangerous because the appliance cannot tell the difference and neither can anyone without test equipment. The failure is a safe one in the sense that it stops rather than fires — the gas valve stays shut. What it will not do is tell you whether it stopped because a wire is shorted or because gas is reaching the burner when it should not be.
Can I keep resetting it?
No, and Alpha limit it anyway. Their own instruction is that if the fault still occurs after pressing reset you contact an approved engineer or Alpha service support, and the appliance counts your attempts — five produces E08, which no further pressing clears. On this particular code repeated resetting is asking an appliance to open a gas valve into a chamber it has just told you it cannot prove is empty.
Why does my Evoke show the same number?
Because it is the same table. Alpha publish an identical error code set for the Evoke and the E-Tec Plus, from E01 to E76, with the same descriptions against each number. What we cannot tell you is whether the older ranges match, because Alpha do not publish those manuals on their current site.
Technical review
Not reviewed by an engineer
No engineer has read this page. Severity follows what the manufacturer publishes, and we would rather tell you that than let a byline imply a sign-off.
Sources and how we verified this
We establish the facts from the sources below and then write the explanation ourselves. We do not copy manufacturer wording and we do not host their manuals.
- Manufacturer technical and service documentation
This page has not been reviewed by an engineer, and we would rather tell you that than imply otherwise. The severity rating follows what the manufacturer publishes: where they say isolate the appliance and call someone, so do we.