What Ariston error 501 means
Error 501 means the boiler could not detect a flame when it tried to light, so it has shut down for safety. Ariston's E-Combi One LPG design, installation and servicing instructions for Great Britain and Ireland list it as "No flame detected", and the Clas One and Clas System One user manual from Ariston Thermo UK lists "No flame detection" in its operation shutdown table. The same code appears in the Clas One installation and servicing instructions for the 18 to 35 models.
Ariston's Spanish help pages describe 501 as an error the boiler cannot correct on its own, which matches how it behaves: the heating and hot water stay off until someone presses RESET.
The manuals print it as "5 01". The 5 places it in the ignition and flame detection group, the same group as the SP1 first ignition warning.
Why 501 needs the RESET button
Ariston splits faults into two kinds. A safety shut-off, such as low pressure, clears automatically once the cause has gone. A shutdown is "non-volatile", meaning it stays latched until you press RESET on the control panel. Error 501 is a shutdown, so even if the gas supply comes back, the boiler will sit there showing 501 until it is reset.
The reason is safety. If a boiler kept trying to light with no flame, unburnt gas could build up. Locking out until a person checks things is a deliberate design choice.
On a Clas One you will see the code together with the display symbol the user manual describes as a request to press RESET (boiler block). The flame symbol, which normally shows when the burner is lit, will be missing.
Before you press RESET: check the gas supply
A reset will only work if the reason for the missing flame has gone. Spend two minutes on these checks first:
- Other gas appliances. Light the hob. If it does not light either, the problem is the supply to the house, not the boiler.
- Prepayment credit. A meter that has run out of credit is one of the most common reasons for a sudden lockout.
- Recent work. If the gas meter was changed or the supply was turned off for work in the street, air in the pipe can stop the boiler lighting at first.
- Isolation valve. The gas valve on the pipe under the boiler should have its handle in line with the pipe.
- LPG level. On an E-Combi One LPG, an empty bottle or a low bulk tank gives exactly this code.
- Frost. In freezing weather, check the outside condensate pipe. If it is frozen, follow our frozen condensate pipe guide before resetting.
If you can smell gas, do not press RESET. Turn off the gas at the meter if it is safe, open doors and windows, leave the property and call the National Gas Emergency Service on 0800 111 999. LPG users should also call their supplier's emergency number.
How to reset Ariston error 501
- Make the checks above and put right anything you find.
- Press the RESET button on the boiler's control panel once.
- Wait several minutes while the boiler goes through its lighting sequence. You may see SP1 or SP2 warnings along the way.
- If 501 comes back, try once or twice more at most.
Ariston's manuals allow up to five resets in 15 minutes. After that, the boiler shows error 304 and will not accept any more resets until the electricity supply is switched off. Hitting that limit tells you resetting is not the answer.
501 compared with other Ariston flame codes
Several codes in the 5 xx group look alike on the display. This is how they differ in the Ariston tables:
| Code | Ariston wording | What it means in practice |
|---|---|---|
| 5 P1 / 5 P2 | 1st / 2nd Ignition Failed | Warnings while the boiler retries |
| 5 P3 | Flame cut-off | The flame lit but went out |
| 5 01 | No flame detected | Lockout after lighting failed |
| 5 02 | Flame detected with gas valve closed | A flame signal when there should be none |
| 5 04 | Flame lift | Unstable flame at the burner |
Codes 5 02 and 5 04 are always engineer jobs. Do not reset either of them repeatedly.
When to call a Gas Safe registered engineer
If the gas supply is fine and 501 returns after two or three resets, the fault is inside the boiler. A Gas Safe registered engineer will typically measure the gas pressure at the boiler, inspect and test the ignition and flame sensing electrodes and their leads, check the spark generator, clear the condensate trap if it is blocked and check the flue. They may also need to test the gas valve and the main PCB if everything else checks out. None of this is safe for a homeowner, so do not remove the casing.
Tell the engineer how many times you have reset the boiler, whether SP1 or SP2 appeared first and whether the fault lines up with cold weather or the first start of the day.
Typical repair costs in the UK
| Job | Typical UK cost |
|---|---|
| Engineer call-out and diagnosis | Typically £60 to £150 |
| Ignition electrode | Typically £90 to £180 |
| Ignition lead or spark generator | Typically £100 to £200 |
| Gas valve replacement | Typically £180 to £350 |
| PCB replacement, fitted | Typically £250 to £550 |
| Flue repair or adjustment | Typically £150 to £400 |