Boiler
104
If your Potterton boiler is showing error code 104, it has detected a flame signal when it shouldn’t be able to — a fault known as ‘ghost flame’ or ‘spurious flame detection’. This is a safety lockout, meaning your boiler has shut itself down to protect your home, and it should be taken seriously. In this guide, we’ll explain exactly what’s causing it, what you can safely do yourself, and when you need to call a Gas Safe registered engineer.
What does this error mean?
Error code 104 on a Potterton boiler — including popular models like the Potterton Assure, Promax, and Titanium ranges — indicates a ‘false flame’ or ‘spurious flame’ fault. In plain English, your boiler’s control board is detecting a flame signal from the flame sensor (also called the ionisation probe) even when the burner should be off or during the pre-ignition phase before gas is supposed to be lit. This triggers an immediate safety lockout. The boiler’s PCB (printed circuit board) is essentially saying: ‘I can see a flame where there shouldn’t be one — I’m shutting down until this is investigated.’ This is a hard lockout fault, meaning the boiler will not restart on its own. It is a safety-critical error and must not be ignored or repeatedly overridden.
Est. repair cost
£80–£150 for an ionisation probe and lead replacement (parts + labour). £250–£500 for a PCB replacement. £150–£350 for a gas valve replacement. Expect a call-out and diagnostic fee of £60–£100 on top if not included.
🔍 Likely causes
🛠️ Step-by-step fix
Step 1 — Do NOT repeatedly reset the boiler. You may attempt a single reset by pressing and holding the reset button (usually marked with a flame or ‘R’ symbol) for 3–5 seconds, as described in your Potterton boiler manual. If the boiler fires up normally and runs without the 104 code returning within a few minutes, monitor it closely over the next 24 hours. If the fault returns, do not reset again — move to the next steps.
Step 2 — Check for obvious external issues. Look around the boiler casing for any signs of water leaks dripping onto the unit, condensation pooling inside, or any loose wiring visible at the casing exterior. Do not open the boiler casing yourself.
Step 3 — Check your gas supply. Confirm other gas appliances in your home (such as your gas hob or gas fire) are working normally. If they are not, contact your gas supplier immediately — a gas supply interruption can sometimes cause unusual boiler behaviour. If your gas supply is fine, the fault is internal to the boiler.
Step 4 — Check the flue terminal (externally). Go outside and look at where the boiler’s flue exits the building. Ensure it is not blocked by debris, a bird’s nest, leaves, or frost. A blocked flue can cause combustion problems that generate unusual flame signals. Do not attempt to modify or clean the flue yourself.
Step 5 — Stop here and call a Gas Safe registered engineer. All remaining causes of error code 104 — including the flame sensor, sensor lead, gas valve, PCB, and burner — are internal gas-related components. UK law (Gas Safety Regulations 1998) prohibits any work on gas appliances by anyone who is not Gas Safe registered. Do not open the boiler casing, do not attempt to clean or replace the ionisation probe yourself, and do not tamper with the gas valve. Book a qualified engineer and tell them: ‘My Potterton boiler is showing error code 104 — a spurious flame fault — and a single reset has not cleared it.’
📞 When to call a professional
You should call a Gas Safe registered engineer as your primary course of action for a Potterton error code 104. This is a gas safety lockout — not a minor inconvenience fault like a low pressure warning. After attempting a single reset (step 1 above), if the fault persists or returns, stop and book an engineer. When you call, tell them: ‘My Potterton boiler [state your model, e.g. Promax Combi 28 HE Plus] is displaying error code 104. It has gone into safety lockout. I’ve tried one reset and the fault has returned. I suspect it may be the flame sensor or possibly the PCB.’ This gives the engineer enough information to arrive prepared with likely parts. Do not use a generic heating engineer — always verify they are Gas Safe registered at gassaferegister.co.uk before booking. If you smell gas at any point, do not attempt any steps — leave the property, do not use light switches, and call the National Gas Emergency Service immediately on 0800 111 999.
⚖️ Repair or replace?
For most Potterton boiler owners, the decision to repair or replace comes down to the age and condition of the boiler. Potterton is a well-established UK brand (part of the BDR Thermea group), and parts for current ranges like the Assure, Titanium, and Promax are generally available. Here’s how to think about it: If your boiler is under 8 years old and has been regularly serviced, a repair is almost certainly the right call. The most common fix — replacing the ionisation probe and its lead — is relatively inexpensive, and even a PCB replacement is cost-effective on a newer boiler. If your boiler is between 8 and 12 years old, get a full assessment from the engineer. Ask them to inspect the heat exchanger and overall condition while they’re there. A repair may still make sense if the boiler is otherwise in good shape. If your boiler is over 12 years old, has not been regularly serviced, or the engineer finds multiple issues beyond the 104 fault (e.g. a corroded heat exchanger or failing pump), replacement is likely more cost-effective. A modern A-rated condensing combi boiler will also reduce your gas bills. Replacement costs in the UK typically range from £1,800 to £3,500 supply and fit, depending on boiler size, brand, and installation complexity. Always get three quotes.