ATAG 501 Fault Code: Causes, Fixes & Repair Costs
What does the ATAG 501 fault code mean?
Fault code 501 on an ATAG boiler means the appliance has tried to ignite and establish a flame multiple times — typically five consecutive attempts — but the ionisation (flame-sensing) circuit has not confirmed a stable flame at any point. The boiler then locks out as a safety measure. The root of the problem is almost always either a failure to produce a flame in the first place (ignition or gas supply issue) or a failure to detect one that is actually present (ionisation sensor or wiring fault). In cold weather, a frozen condensate pipe can also trigger this exact code by disrupting the boiler's operation before ignition completes.
General guidance only — not a substitute for professional advice, and not certified by a Gas Safe engineer. Always have your specific appliance assessed by a qualified professional. Any gas work must be carried out by a Gas Safe registered engineer. If you smell gas or suspect carbon monoxide, leave the property and call the National Gas Emergency line on 0800 111 999.
Common causes
- Faulty or dirty flame-sensing (ionisation) electrode Common
The ionisation electrode sits in the burner assembly and confirms to the PCB that a flame is present by passing a tiny electrical current through it. If the tip is coated in carbon deposits, corroded, or cracked, it cannot send a reliable signal even when the burner is alight. This is the most frequently identified culprit when the boiler produces a spark but still locks out on 501.
- Blocked or partially obstructed burner / pilot jet Common
Soot, debris, or scale can partially block the gas ports on the burner or pilot jet, preventing enough gas from flowing to sustain a flame that the sensor can detect. This is especially likely on boilers that have not been serviced regularly.
- Gas supply interruption Common
If the gas meter valve is partially closed, a prepayment meter has run out of credit, or there is a wider supply interruption from the network, the boiler will attempt ignition against a very weak or absent gas supply and repeatedly fail. Other gas appliances in the property not working is a quick confirmation of this.
- Frozen condensate pipe Sometimes
During cold spells, the condensate drain pipe — particularly any section running outside — can freeze solid. The resulting backpressure or flow restriction causes the boiler to fault before it completes a successful ignition cycle, and ATAG specifically list codes 133 and 501 as the codes that appear in this scenario.
- Faulty ignition electrode or lead Sometimes
A worn, cracked, or incorrectly gapped ignition electrode may produce a weak spark that is insufficient to light the gas reliably. A damaged or poorly routed HT lead can also cause the spark energy to arc to earth rather than across the electrode gap.
- Gas valve not opening correctly Sometimes
The gas valve is an electromechanical component that opens on command from the PCB to allow gas to the burner. If the valve coils are weakening, the valve is sticking, or wiring connections to it are corroded, insufficient gas reaches the burner and ignition fails repeatedly.
- PCB fault Rare
The printed circuit board manages the entire ignition sequence and reads the ionisation signal. If the PCB is misreading the sensor signal or failing to fire the ignition at the right moment, the boiler will lock out with 501. This is typically only suspected once all other components have been checked and found to be in order.
How to fix it
- Check that your gas supply is active DIY safe
See whether other gas appliances in your home — such as a gas hob or fire — are working normally. If nothing gas-powered is working, contact your gas supplier or check whether a prepayment meter needs topping up. If your supply is fine, move on to the next step.
- Reset the boiler (up to two or three times) DIY safe
Locate the reset button on the boiler's control panel — on most ATAG models this is a clearly labelled button or a dedicated function on the display. Hold it for the period stated in your user manual. If the boiler fires up and runs normally, monitor it over the next day. If it locks out again, do not keep resetting repeatedly — further resets will not fix an underlying fault and could mask a safety issue.
- Check for a frozen condensate pipe in cold weather DIY safe
In freezing temperatures, inspect the white or grey plastic condensate pipe where it exits the property and runs outdoors. If it feels solid or you can see ice, thaw it gently by applying a warm (not boiling) water bottle or warm damp cloths along the frozen section, working from the drain end upward. Once thawed, reset the boiler. Insulating the pipe afterwards helps prevent a recurrence.
- Do not attempt to open the boiler casing or touch internal components Gas Safe engineer
Accessing the burner, electrodes, ionisation sensor, gas valve, or any internal gas components requires Gas Safe registration by law. Even if you are confident around appliances, working on gas systems without registration is illegal and potentially fatal. Leave all internal checks to a qualified engineer.
- Contact ATAG directly if your boiler is under warranty DIY safe
ATAG boilers carry a 10-year standard warranty. If yours is within that period, call ATAG Heating UK on 0800 680 0100 before arranging an independent engineer — ATAG dispatch their own engineers for warranty faults and using an unauthorised repairer may affect your cover.
- Book a Gas Safe registered engineer to diagnose and repair the fault Gas Safe engineer
If the boiler continues to lock out after the checks above, a Gas Safe engineer will need to inspect the ionisation electrode and its wiring for carbon fouling, corrosion, or damage; test the ignition electrode and HT lead; verify the gas valve is opening and delivering the correct pressure; and, if all else checks out, assess the PCB. Parts are then cleaned or replaced as needed. Ask the engineer to service the boiler at the same time if it has not been done in the past 12 months, as this can prevent the 501 code recurring.
Parts you may need
- Ionisation / flame-sensing electrode · from £35
- Ignition electrode · from £30
- Ignition HT lead · from £20
- Gas valve · from £120
- PCB (printed circuit board) · from £220
The exact spare depends on your boiler's GC number (on the data badge). Check this against the part before buying.
Typical repair cost
Expect to pay roughly £120–£350, depending on the underlying cause.
Frequently asked questions
Can I fix ATAG fault code 501 myself by just resetting the boiler?
A reset is always the sensible first step and will occasionally clear a transient fault, but if the boiler locks out again within a short time the underlying cause — a dirty sensor, ignition problem, or gas supply issue — has not been resolved. Repeatedly resetting a boiler that keeps faulting can delay diagnosis and, in rare cases, mask a genuine safety concern. Limit yourself to two or three resets and call a Gas Safe engineer if the fault returns.
Why does ATAG code 501 appear in cold weather even though the boiler was working fine?
In cold snaps, the condensate pipe that carries acidic water away from the boiler can freeze, especially where it runs outside or through an unheated space. ATAG specifically identify this as a trigger for the 501 code. Thawing the pipe (using warm water or a hot-water bottle — never a blowtorch) and then resetting the boiler usually resolves it. If the same pipe freezes every winter, ask an engineer to lag or re-route it.
How much does it usually cost to repair ATAG fault code 501 in the UK?
For the most common repairs — cleaning or replacing the ionisation electrode, ignition electrode, or HT lead — expect to pay roughly £120–£350 including labour. A gas valve replacement sits towards the higher end of that range. A PCB replacement is a more significant outlay, typically £350–£500 or more including parts and labour; if your boiler is old and out of warranty, an engineer may suggest comparing that cost against a new boiler installation. Annual servicing at around £80–£120 is the most cost-effective way to avoid the fault altogether.
Does ATAG fault code 501 mean I have a gas leak?
Not directly — the code means the boiler failed to detect a flame, not that gas is escaping unburned. However, if at any point you smell gas, do not attempt to reset the boiler. Turn off the gas at the meter, open windows, leave the property, and call the National Gas Emergency Service on 0800 111 999 immediately. A Gas Safe engineer must check the installation before the boiler is used again.