Boiler
108
Error code 108 on a Potterton boiler indicates a flame sensor or ignition fault — the boiler is trying to fire up but cannot detect a stable flame, so it shuts down as a safety precaution. While this can feel alarming, especially in cold weather, it is one of the more common Potterton faults and is not always expensive to fix. In this guide, you will learn exactly what causes error code 108, what you can safely check yourself, and when you need to call a Gas Safe registered engineer.
What does this error mean?
Error code 108 on Potterton boilers — including popular models such as the Potterton Promax, Performa, and Gold range — is a lockout fault related to flame failure. When your boiler fires up, a component called the flame sensor (also known as an ionisation probe) checks within a few seconds that a stable flame has been established at the burner. If it cannot confirm this — either because ignition has failed, the flame has gone out unexpectedly, or the sensor itself is faulty — the boiler’s control board triggers error code 108 and locks the unit out for safety. This is a built-in protection feature: a boiler that cannot reliably confirm combustion should not continue attempting to run, as unburned gas is dangerous. The boiler will typically attempt to ignite two or three times before locking out and displaying the fault code on the display panel.
Est. repair cost
£80–£150 for a Gas Safe engineer call-out and flame sensor or electrode replacement; £200–£450 if the gas valve requires replacement; £300–£550+ if the PCB is at fault. Parts alone: flame sensor £15–£40, ignition electrode £10–£30, gas valve £80–£180.
🔍 Likely causes
🛠️ Step-by-step fix
Step 1 — Perform a manual reset: Locate the reset button on your Potterton boiler (usually a dedicated button on the front panel, sometimes marked with a flame or ‘R’ symbol). Press and hold it for 3–5 seconds, then release. Wait for the boiler to complete its ignition sequence. If it fires up successfully and stays running, monitor it over the next 24 hours. A one-off lockout can be caused by a momentary gas supply fluctuation and may not reoccur.
Step 2 — Check your gas supply: Go to your kitchen and turn on a gas hob burner (if you have one). If it lights normally with a strong, steady flame, your gas supply is likely fine. If it does not light at all, or the flame is very low and weak, contact your gas supplier immediately — there may be a supply issue in your area. Do not attempt to use the boiler until gas supply is confirmed normal.
Step 3 — Check your gas meter: Make sure your gas meter is working correctly and has not been inadvertently switched off. Check that the handle on the emergency control valve (usually located next to the meter) is aligned parallel to the pipe, meaning it is open. If it is perpendicular (across the pipe), it is closed — turn it to the open position.
Step 4 — Check for other gas appliances working: Quickly check that any other gas appliances in the home (gas fire, gas cooker) are working normally. If all gas appliances are failing, the problem is with your supply, not the boiler itself. Call your gas supplier’s emergency line (National Gas Emergency Service: 0800 111 999).
Step 5 — Inspect the flue outlet externally: Go outside and locate the flue terminal on the exterior wall of your property (it looks like a white or grey plastic vent, usually low on the wall). Check that it is not visibly blocked by a bird’s nest, leaves, debris, or any object that may have been placed in front of it. Do not insert anything into the flue. If it appears blocked, you will need a Gas Safe engineer to clear it safely.
Step 6 — Reset once more after checks: After completing the above checks, attempt one more reset as described in Step 1. If the boiler fires up and holds, you are likely dealing with an intermittent fault. Make a note of the date, frequency, and circumstances of the lockouts — this information is valuable for a Gas Safe engineer if the fault returns.
Step 7 — Do not attempt further DIY: If the boiler locks out again with error code 108 after two reset attempts, or if it will not reset at all, do not continue attempting further resets. All remaining causes — including cleaning or replacing the flame sensor, testing or replacing ignition electrodes, inspecting the gas valve, and diagnosing the PCB — require a Gas Safe registered engineer. Working on gas components without Gas Safe registration is illegal under the Gas Safety (Installation and Use) Regulations 1998 and is extremely dangerous.
📞 When to call a professional
You should call a Gas Safe registered engineer if: the boiler locks out again after one or two successful resets; the boiler will not reset at all; you can see or smell any sign of gas; the flue terminal appears blocked or damaged; or you have any doubt whatsoever about the cause. This is a gas appliance — beyond the basic checks listed above, all diagnostic and repair work must be carried out by a Gas Safe registered engineer by law. When you call, tell the engineer: (1) the exact error code displayed — 108; (2) the make and model of your boiler (e.g. Potterton Promax 28HE Plus); (3) how many times you have attempted a reset and whether it fired briefly before locking out again; and (4) whether any other gas appliances in the home are affected. This information will help the engineer diagnose the fault before they arrive and may save time and cost on the visit. You can find a Gas Safe registered engineer at gassaferegister.co.uk or by calling 0800 408 5500.
⚖️ Repair or replace?
For most Potterton boilers displaying error code 108, repair is the right first step — provided the boiler is under 10 years old and has been reasonably well maintained. The most common fix (cleaning or replacing the flame sensor or ignition electrodes) is a relatively straightforward job for an experienced engineer and is not prohibitively expensive. Parts for Potterton boilers are widely available in the UK through merchants such as Plumb Center and national boiler parts suppliers, so you are unlikely to face long delays. However, if the fault has been caused by a failed gas valve or a faulty PCB, the repair cost rises significantly. At that point, the age and condition of the boiler becomes crucial. As a general rule: if your Potterton boiler is over 10–12 years old and the repair quote exceeds £400–£500, seriously consider replacement. Modern A-rated condensing boilers (from Baxi, Ideal, or Worcester Bosch) will be significantly more energy-efficient than an older Potterton, and with energy bills at current UK levels, the payback period on a new boiler can be surprisingly short. A new boiler typically costs £1,800–£3,000 installed, depending on output, brand, and location. If your boiler is under 8 years old and the fault is a sensor or electrode, repair is almost always the sensible choice.