What the A01 code means
When your boiler gets a call for heating or hot water, it runs a start-up sequence. The fan starts, the gas valve opens, and an ignition spark lights the gas at the burner. A flame sensing electrode then confirms that a flame is present. If the boiler does not sense a flame within its safety time, it closes the gas valve and locks out.
On most current Vokera models, including the Vision, Vision Plus, Evolve, Easi-Heat Plus and Unica, the user manual lists A01 as ignition failure, flame not sensed or internal fault. It is a final lockout, which means the boiler stays off until it is reset. The manual's action for A01 is to call an engineer, but there are some simple outside causes that are worth ruling out first, because they are common and cost nothing to fix.
A01 is a safety feature. The boiler is stopping unburnt gas from building up, which is exactly what it should do.
Common causes of A01
No gas reaching the boiler
If the gas supply to the house has been interrupted, or the isolation valve under the boiler has been turned off, the boiler has nothing to light. Low gas pressure can have the same result.
Prepayment meter out of credit
This is one of the most frequent causes. Once the credit and emergency credit run out, the gas stops. The boiler shows A01 the next time it tries to fire.
Frozen or blocked condensate pipe
Condensing boilers drain waste water through a plastic pipe that often runs outside. In a hard frost it can freeze, water backs up, and many boilers lock out on an ignition fault as a result. A gurgling sound from the boiler is a common clue.
Ignition or flame sensing electrode
The electrodes wear over time. A dirty, worn or badly positioned electrode can fail to spark properly or fail to sense a flame that is present.
Gas valve, fan or control board
Less commonly, the gas valve does not open, the control board does not send the right signals, or there is an internal fault that stops the boiler lighting. These need testing by an engineer.
| Cause | Typical sign | Who fixes it |
|---|---|---|
| Prepayment credit run out | Hob or gas fire will not light either | You, by topping up |
| Gas isolation valve closed | Handle across the pipe rather than in line | You, by opening it |
| Frozen condensate pipe | Freezing weather, gurgling from the boiler | You, by thawing the pipe |
| Worn or dirty electrode | Gas supply fine, fault keeps returning | Gas Safe engineer |
| Gas valve or PCB fault | Repeated lockouts with a good gas supply | Gas Safe engineer |
Safe checks you can do yourself
- Check your other gas appliances. If the hob or gas fire will not light, the problem is the gas supply, not the boiler. Contact your gas supplier.
- If you have a prepayment meter, check the credit and top up. Some meters need a button pressing to restore the supply after a top-up.
- 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 look 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.
- Reset the boiler using the method in your manual. On many models you press and hold the reset button for a few seconds, and on some you turn the selector dial to the reset position.
- Give it a few minutes to run through the start-up sequence. Stop after two or three attempts.
If you smell gas, do not reset the boiler or use 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 your gas supply is working, the condensate pipe is clear and A01 comes back after two or three resets, the cause is inside the boiler. Call a Gas Safe registered engineer. They will check the gas pressure at the boiler, inspect the electrodes and leads, test the gas valve and look at the flue and control board.
When you book, give the exact code, the model name from the front of the boiler, and say whether your other gas appliances work and whether the fault started in cold weather. That helps them bring the right parts. Never remove the casing or try to clean or adjust the electrode yourself.
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, including parts and labour.
| Job | Typical UK cost |
|---|---|
| Engineer call-out and diagnosis | Typically £80 to £150 |
| Ignition or flame sensing 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 A01 coming back
Keep prepayment credit topped up, especially in winter when gas use climbs quickly. If your condensate pipe runs outside, ask an engineer whether it can be insulated, enlarged or rerouted, as this is often the simplest way to avoid winter lockouts. A yearly service lets the engineer check and replace worn electrodes before they fail. Our boiler service cost guide explains what a service includes. If your boiler shows a pressure fault instead, see our page on the Vokera A04 low pressure code, and the Vokera fault code hub covers the rest.