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Error 501 · Gas boilers
Error 501: the boiler fails to light
The boiler has tried to light three times without detecting a flame and has locked out for safety. It may be a lack of gas, but it can also be a fault in the ignition system.
What it means
Every time you call for heating or hot water, the boiler opens the gas valve, creates a spark at the burner and uses the ionisation electrode to check that a flame has formed.
If it doesn't detect a flame, it shuts off the gas and tries again. After three failed attempts, it locks out and shows error 501. This is a safety lockout: it won't try again until you reset it.
Most likely causes
- No gas. The shut-off valve is closed, the supply has been cut off or, if the installation runs on propane or butane, the gas bottle or tank is empty.
- Air in the gas pipe. This can happen after changing the gas bottle or after building work: the first attempts may fail until the gas reaches the burner.
- Ignition or ionisation electrodes that are dirty, worn or out of position: the spark doesn't jump properly or the boiler fails to detect the flame.
- Insufficient gas pressure at the boiler inlet, due to the regulator or the installation.
- Gas valve or circuit board. These can also cause the lockout.
What you can check yourself
Without tools and without opening the unit.
- Check whether there's gas in the rest of the house. If you have a gas hob, light a ring. If there's no gas there either, the problem is most likely in the supply, not the boiler.
- Check the boiler's gas valve, which is usually underneath. With the handle in line with the pipe, it's open; across the pipe, it's closed.
- If you use a gas bottle or tank, check there's gas left.
- Reset the boiler with the gas turned on: press the reset button and wait for it to finish its ignition attempts.
If it locks out again after two or three resets, don't keep trying: book an engineer. And if you smell gas, don't reset it: follow the instructions below.
If the water heater, boiler or heat pump is leaking a lot of water: turn off the main water stopcock in your home and call us on 871 559 176. That stops the leak and saves you from water damage.
If you smell gas: turn off the gas at the shut-off valve and open the windows. Don't touch any switches or plugs, and call us from outside the property.
If the unit is smoking or getting hotter than it should: switch it off, disconnect it from the mains and call us.
If we come out
What we check and how to stop it happening again
What the engineer usually checks
- Gas pressure at the boiler inlet, at rest and with the burner running
- Condition and position of the electrodes
- Ionisation current: the signal the boiler uses to detect the flame
- Gas valve and circuit board
- A combustion analysis at the end
How to catch it early
Electrodes get dirty and wear with use. At the annual service, the burner and combustion chamber are cleaned and a combustion analysis is carried out: that way, many ignition faults that aren't down to the supply can be caught early.
See the maintenance plansStill not sorted?
Book an engineer and give us the code and the model of your unit. That way we know what to bring.