Boiler
120
Error code 120 on a Potterton boiler indicates a flame detection or ignition lockout fault — the boiler has tried to light and failed, shutting itself down as a safety measure. This needs prompt attention, especially in cold weather, but a simple reset may resolve it.
What does this error mean?
When your Potterton boiler displays error code 120, it means the boiler’s control board has detected that the burner failed to ignite or the flame was not confirmed by the flame-sensing electrode within the allowed time. The boiler locks out to prevent unburned gas from accumulating — a critical safety function. You’ll have no heating or hot water until the fault is cleared.
Est. repair cost
£100–£350 for a Gas Safe engineer call-out, diagnosis, and repair (electrode, igniter, or gas valve replacement). A PCB replacement can reach £400–£500.
🔍 Likely causes
🛠️ Step-by-step fix
Check your gas supply first: try lighting a gas hob or checking with your supplier. If there’s no gas to your home, call your supplier — not an engineer.
Check for a frozen condensate pipe: if temperatures are below freezing, locate the white plastic pipe running outside and carefully thaw it with warm (not boiling) water and a hot water bottle.
Perform a boiler reset: press and hold the reset button (usually marked with a flame symbol) for 3–5 seconds until the boiler attempts to relight. Wait 10 minutes. If it fires up and stays on, monitor it closely.
Reset once more if the first attempt fails — some lockouts require two resets. Do not reset more than twice; repeated lockouts mean there is an underlying fault that needs diagnosis.
Check your gas meter hasn’t tripped to emergency credit if you’re on a prepayment meter — top it up if needed.
If the boiler locks out again after resetting, stop here and call a Gas Safe registered engineer. Do not attempt to open the boiler casing, adjust the gas valve, or interfere with any internal components — this is illegal without Gas Safe registration and dangerous.
📞 When to call a professional
Call a Gas Safe registered engineer if the boiler locks out again after two resets, or if you smell gas at any point — in which case call the National Gas Emergency line on 0800 111 999 immediately. Tell the engineer you have a Potterton error code 120 (ignition lockout), how many times it has locked out, and whether it’s been happening intermittently or is constant — this helps them bring the right parts.
⚖️ Repair or replace?
For a boiler under 10 years old, repair is almost always the right call — replacing a flame electrode or gas valve typically costs £150–£350 including labour, which is far cheaper than a new boiler. If your Potterton boiler is over 12–15 years old and this fault is accompanied by other recurring issues, a replacement (typically £2,000–£3,000 installed for a modern combi) is worth serious consideration. A Gas Safe engineer can give you an honest assessment on-site.