What the F28 code means
On most ecoTEC models, F28 means the boiler has failed during ignition on start-up. When the boiler gets a call for heat, the fan starts, the gas valve opens and the ignition electrode sparks to light the gas. A flame sensor then checks that a flame has formed. If no flame is detected after several attempts, the boiler shuts the gas off and locks out with F28.
This is a safety lockout. The boiler is stopping unburnt gas from building up, which is exactly what it should do. F28 does not always mean a part has failed. Very often the boiler simply is not getting gas, and that is something you can check yourself in a few minutes.
Common causes of F28
Gas supply off or low
If the gas isolation valve under the boiler has been knocked closed, or the supply to the house has been interrupted, the boiler cannot light. Low gas pressure, for example when a meter or supply pipe is undersized, can cause the same result.
Prepayment meter out of credit
This is one of the most common causes we hear about. If the credit or emergency credit has run out, the gas stops and the boiler shows F28 the next time it tries to fire.
Frozen or blocked condensate pipe
In freezing weather the condensate pipe can freeze where it runs outside. Water then backs up inside the boiler, and on many ecoTEC models this leads to repeated lockouts that can show as F28. A gurgling noise from the boiler is a common clue.
Ignition electrode or lead
The electrode wears over time and can become dirty or move out of position. A damaged ignition lead can also stop the spark reaching the burner.
Gas valve or control board
If the gas valve does not open properly, or the main control board is not sending the right signal, the boiler cannot light. These faults need proper testing by an engineer.
| Cause | Typical sign | Who fixes it |
|---|---|---|
| Prepayment credit run out | Hob also will not light | You, by topping up credit |
| Gas valve under boiler closed | Handle across the pipe, not in line | You, by opening the valve |
| Frozen condensate pipe | Freezing weather, gurgling from boiler | You, by thawing the pipe |
| Worn or dirty ignition electrode | Clicking but no flame, fault keeps returning | Gas Safe engineer |
| Faulty gas valve or control board | Gas supply fine, repeated lockouts | Gas Safe engineer |
Safe checks you can do yourself
- Check your other gas appliances. If the hob or gas fire will not light either, the problem is the gas supply, not the boiler.
- If you have a prepayment meter, check the credit and top up if needed. Some meters need a button pressing to restore the supply afterwards.
- Check the gas isolation valve on the pipe under the boiler. The handle should be in line with the pipe, which means it is open.
- In freezing weather, find the condensate pipe where it leaves the house and check for ice. Thaw it with warm (not boiling) water or a hot water bottle. Our frozen condensate pipe guide shows how.
- Check the thermostat and timer are calling for heat, so the boiler is actually trying to fire.
- Press reset as your manual describes, and wait a few minutes. Try no more than two or three times.
If you smell gas, do not press reset or operate electrical switches. Turn off the gas at the meter if it is safe to do so, open doors and windows, leave the property and call the National Gas Emergency Service on 0800 111 999.
When to call a Gas Safe engineer
If the gas supply is fine, the condensate pipe is clear and F28 comes back after two or three resets, the cause is almost certainly inside the boiler. Call a Gas Safe registered engineer. They will check the gas pressure at the boiler, the ignition electrode and lead, the gas valve and the flue, and look for any stored fault history on the control board.
When you book, give the exact code, say whether your other gas appliances work, and mention if the fault started during cold weather. That information points the engineer towards the right part and can save a second visit, which keeps the overall cost down.
Do not remove the casing or try to clean or adjust the electrode yourself. Work on gas components is legally restricted to Gas Safe registered engineers for good reason.
Typical repair costs in the UK
If the fix was a gas top-up or a thawed pipe, there is no cost. If a part has failed, these ranges are typical. Your final bill depends on your area, the engineer and the exact model.
| Job | Typical UK cost |
|---|---|
| Engineer call-out and diagnosis | Typically £80 to £150 |
| Ignition electrode replacement | Typically £100 to £180 |
| Ignition lead replacement | Typically £80 to £150 |
| Gas valve replacement | Typically £250 to £450 |
| Rerouting or insulating the condensate pipe | Typically £100 to £250 |
How to stop F28 coming back
Keep prepayment credit topped up, particularly in winter when gas use rises quickly. If your condensate pipe runs outside, ask an engineer whether it can be insulated or rerouted, as this is often the simplest way to avoid winter lockouts. A yearly service lets an engineer check and replace a worn electrode before it causes a breakdown. Our boiler service cost guide explains what is included. If your boiler lights but then cuts out after a few seconds or minutes, see our page on the Vaillant F29 flame loss fault instead.