Alpha E20 fault codewhat it means and how to fix it
The appliance is having trouble sensing the flame it has lit. It usually points at the sensing electrode or its earth rather than at the gas side.
Take care
Get a Gas Safe registered engineer to look at your Alpha E20
Gas Safe Register engineers only. No obligation.
What E20 means
Flame sensing on a domestic boiler works by passing a tiny current through the flame itself — a flame conducts, and the appliance measures that conduction to prove the burner is alight. E20 means that measurement is unreliable: the appliance is seeing a signal, but not a clean or strong enough one to trust. Because the current involved is measured in microamps, this fault is very sensitive to things that would not trouble any other circuit. A film of oxide on the electrode tip, a poor earth on the appliance chassis, or a slightly out-of-position electrode will all degrade the signal. That is genuinely good news: the most common fixes here are cheap. It also explains why E20 often appears intermittently for weeks before becoming permanent, and why it is more common in properties with older or marginal electrical earthing.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Dirty or oxidised sensing electrode | A film builds on the tip and reduces the current the appliance can read. Cleaning is sometimes enough, though replacement is the honest fix once it has started. | Common |
| Poor appliance earth | Flame rectification depends on a good earth path. A loose earth at the fused spur or a corroded terminal will produce this exactly. | Occasional |
| Electrode out of position | The tip has to sit in the flame envelope. If it has been knocked during a previous service, or the bracket has distorted with heat, the signal drops. | Occasional |
| Control board fault | The amplifier stage on the board that reads the ionisation current has degraded. Uncommon, and only diagnosed once the cheap causes are eliminated. | Rare |
Can you fix it yourself?
Stop — this needs an engineer
What it costs to fix
Typical repair cost
£110–£380
An electrode replacement is £110 to £190 fitted. An earthing fault is usually a call-out charge plus a few pounds of materials. A control board is £300 to £480.
Typical UK prices including parts and labour, updated August 2026. Your quote may differ.
What other owners found
Nobody has shared what worked for them on this code yet. If you have had it, you would be the first — and it genuinely helps the next person.
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
- EA 227Stop — call an engineerThe boiler tried to light and never detected a flame, so it has shut the gas off and locked out. This is the most common Worcester fault code.Related: ignition and gas supply
- F.28Stop — call an engineerThe boiler tried to light and failed three times in a row, so it has shut the gas off and locked out. Do not keep resetting it.Related: ignition and gas supply
- 501Get it checkedNo flame was detected during ignition and the appliance has locked out. Check the gas supply and the condensate pipe before booking anyone.Related: ignition and gas supply
- A01Get it checkedThe burner failed to light and the appliance has locked out. Check the gas supply and, in cold weather, the condensate pipe before calling anyone.Related: ignition and gas supply
Related guides
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
- Interviews with working engineers
Checked by Technical reviewer (Gas Safe registration to be displayed on appointment) on 5 August 2026.