What the F62 code means
When your boiler finishes a heating cycle, the circuit board closes the gas valve and expects the flame to go out almost immediately. It confirms this using the same electrode that watches for a flame during normal running. If the flame signal lingers for longer than it should, the board cannot be sure the gas has really stopped, and it shuts the boiler down.
On most ecoTEC models, Vaillant's installation manual lists F62 as a gas valve switch-off delay. The possible causes it gives are a delayed fading of the flame signal picked up by the ignition and monitoring electrode, a defective gas valve, or a defective PCB. In other words, either the gas valve is slow to close, or the parts that measure the flame are giving a misleading reading.
F62 is closely related to F27 (flame simulation). Both involve the boiler seeing a flame it does not expect. F27 is triggered when a flame signal appears while the gas should be off, while F62 is about the flame not disappearing quickly enough at shutdown.
Common causes of F62
A gas valve that closes slowly
Gas valves are mechanical as well as electrical. Wear, dirt or a weak spring can make the valve close a fraction too slowly, allowing a small amount of gas to keep burning. This is the cause an engineer will want to rule in or out first, because it affects safety.
A worn or dirty flame detection electrode
Deposits on the electrode, or an electrode that has shifted out of position, can hold on to a signal for a moment after the flame has actually gone. The board then reads that as a delay.
A faulty PCB
If the flame monitoring circuit on the board is failing, it may be slow to recognise that the flame has gone out. Vaillant lists a defective PCB as a possible cause.
Wiring or connection faults
Loose or corroded connections between the electrode and the board can also distort the signal, particularly if moisture has got into the boiler.
| Cause | Typical sign | Who fixes it |
|---|---|---|
| Slow-closing gas valve | F62 at the end of heating or hot water cycles | Gas Safe engineer |
| Dirty or misplaced electrode | Intermittent F62, sometimes with ignition faults | Gas Safe engineer |
| PCB flame monitoring fault | F62 alongside other unexplained lockouts | Gas Safe engineer |
| Corroded connections | F62 after damp conditions or a leak | Gas Safe engineer |
Safe checks you can do yourself
Because every likely cause of F62 is inside the boiler, homeowner checks are limited. Their main value is ruling out anything obvious and giving the engineer a clear picture:
- Note the code and when it appears. Does it happen when the heating switches off, when a hot tap is turned off, or at random?
- Check for any drips or damp patches under the boiler, without removing any covers.
- Check the fused spur is on and the power supply is steady.
- Check that your thermostat and timer settings are normal, so you can tell the engineer whether the boiler is switching on and off unusually often.
- Press reset once, following your manual. If F62 comes back, you can try once more, but after that leave the boiler off.
A gas valve that does not close promptly is a safety issue. Do not keep resetting the boiler to get it working. If you smell gas, turn off the supply at the meter if it is safe to do so, open windows, leave the property and call the National Gas Emergency Service on 0800 111 999.
When to call a Gas Safe engineer
Call a Gas Safe registered engineer whenever F62 returns after a reset. Tell them the exact code, the boiler model and when the fault tends to happen. That timing clue is especially useful with F62, because it can point towards either the gas valve or the detection side.
A good engineer will inspect the electrode and its lead, test the gas valve for correct operation and tightness, check the gas pressure and look at the flame monitoring on the board. They should always confirm the gas valve is shutting off properly before deciding the board is at fault. After any part is replaced, they should check combustion and make sure the boiler runs through several heating cycles without the code returning.
If the engineer finds the gas valve is not closing properly, they may leave the boiler switched off and labelled until the part is replaced. That is standard safety practice, not an overreaction.
Typical repair costs in the UK
Costs depend on which part has failed. These typical ranges include parts and labour but will vary by area.
| Job | Typical UK cost |
|---|---|
| Engineer call-out and diagnosis | Typically £80 to £150 |
| Cleaning or replacing the electrode | Typically £90 to £180 |
| Replacing the gas valve | Typically £200 to £400 |
| Replacing the main PCB | Typically £250 to £500 |
How to stop F62 coming back
Regular servicing is the best defence against F62. During a service, an engineer checks the electrode, looks at the burner and tests the gas valve and combustion, which is exactly where F62 faults start. Our boiler service cost guide explains what a service includes and what it typically costs. Keep the boiler dry by getting leaks fixed quickly, and if you have also seen F61 or F27, mention it to the engineer, as those codes involve the same group of parts. You can find other ecoTEC codes on our Vaillant fault codes page.