ATAG 129 Fault Code: Causes, Fixes & Repair Costs
What does the ATAG 129 fault code mean?
Fault code 129 on ATAG boilers indicates that the boiler's fan has failed to start up. When the fan cannot spin up to the required speed, the boiler locks out as a safety precaution and refuses to fire. You may notice a bell symbol displayed alongside the code 129 on the boiler — this confirms the unit is in lockout and needs a reset and/or engineer intervention to clear. If you have an ATAG digital programmer plugged in, it will display a sub-code alongside 129 that points to the specific nature of the fan fault — for example, sub-code 2 indicates 'fan failure — low RPM', meaning the fan is spinning but cannot reach the speed required for safe combustion. A related but separate code, 151, covers situations where the fan starts but cannot maintain or achieve the correct speed range, or where there is a control unit fault — so if you are seeing 151 rather than 129, the fan itself may be partially operational but not reliably controlled.
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
- Fan motor failure or seizure Common
The fan motor itself has worn out or seized mechanically and cannot spin up at all. This is the most common root cause of a 129 lockout and typically requires a full fan assembly replacement.
- Physical obstruction around the fan Sometimes
Debris, dust build-up, or a foreign object caught in the fan blades can prevent the motor from turning. An engineer will inspect the fan housing and clear any blockage found.
- Wiring or connector fault between PCB and fan Sometimes
Corroded, loose, or damaged wiring between the printed circuit board and the fan motor can stop the start signal reaching the fan, or prevent feedback from reaching the PCB. A multimeter check of the wiring harness will confirm or rule this out.
- Failed fan capacitor Sometimes
Some ATAG fan assemblies use a start capacitor to give the motor its initial kick. If this component fails, the motor may hum but not rotate, producing a 129 fault.
- Worn fan motor bearings Sometimes
Bearing wear causes increasing mechanical resistance over time, eventually preventing the motor from starting. This is more common on older boilers and is usually accompanied by unusual noise before full failure.
- PCB fault preventing fan activation Rare
If the PCB cannot send or receive the correct signals to drive the fan, the boiler will lock out with code 129 even if the fan itself is undamaged. PCB faults are less common but should be considered if the fan tests as electrically sound.
How to fix it
- Check your gas supply is working DIY safe
Confirm other gas appliances in the property — such as a gas hob — are working normally. If you have no gas supply at all, contact your gas supplier rather than an engineer.
- Attempt a boiler reset DIY safe
Press and hold the reset button on the boiler control panel as described in your user guide. The boiler will attempt to restart. If it locks out again with code 129, do not reset more than two or three times in quick succession as this can cause further issues. A persistent 129 after resetting confirms an engineer is needed.
- Inspect the external flue terminal for obvious blockage DIY safe
From outside, visually check that the flue outlet and air intake are not obstructed by leaves, bird nesting material, or other debris. Do not attempt to dismantle anything — this is a visual check only. If you spot a blockage you cannot safely remove from outside, report it to your engineer.
- Engineer to test fan wiring and connections Gas Safe engineer
A Gas Safe engineer will use a multimeter to check continuity and voltage across the wiring harness between the PCB and the fan motor. Loose, corroded, or broken connectors will be tightened or replaced at this stage.
- Engineer to inspect and test the fan motor and capacitor Gas Safe engineer
The engineer will physically inspect the fan blades for obstructions and bearing wear, then electrically test the motor and any associated capacitor. If the motor is seized or the capacitor has failed, the relevant component will be replaced.
- Engineer to test PCB output signals Gas Safe engineer
If the fan motor and wiring both test as sound, the engineer will check whether the PCB is correctly sending drive signals to the fan. A faulty PCB will need repair or replacement by a qualified professional.
- Call a Gas Safe registered engineer if the fault persists Gas Safe engineer
All internal boiler work on gas appliances is legally required to be carried out by a Gas Safe registered engineer in the UK. If your boiler is still within its warranty period, contact ATAG directly on 0800 680 0100 before arranging a third-party repair, as warranty work is carried out by ATAG's own engineers at no charge.
Parts you may need
- Fan assembly (ATAG iC compatible) · from £130
- Fan start capacitor · from £15
- Wiring harness / loom connector set · from £25
- PCB (printed circuit board) · from £280
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 129 myself?
The only safe steps a homeowner can take are checking the gas supply is on, looking at the external flue for obvious blockages, and attempting a reset once or twice. Everything else — testing the fan motor, replacing components, or working on wiring inside the boiler — must be carried out by a Gas Safe registered engineer. Attempting internal boiler work without registration is illegal in the UK and can be dangerous.
How much does it cost to fix a 129 fault on an ATAG boiler?
For most people, the repair falls somewhere between £120 and £350 depending on what is found. A wiring or connection fix is typically at the lower end, while a full fan assembly replacement usually comes in around £225–£275 all-in including labour. If the PCB is also at fault, costs can rise considerably — PCB replacement on ATAG boilers tends to be in the region of £400–£500 — though this is less common. If your boiler is approaching 10–15 years old and needs a major component, it is worth getting a new boiler quote alongside the repair quote.
What is the difference between ATAG fault codes 129 and 151?
Both codes relate to the fan but indicate different failure modes. Code 129 means the fan has not started at all — the boiler cannot detect any rotation when it tries to fire. Code 151 means the fan starts but cannot reach or maintain the correct speed range, or there is a control unit fault affecting speed regulation. In practice, 129 is more often a mechanical or electrical failure of the fan motor or its wiring, while 151 can sometimes point toward a PCB or speed-control issue. An engineer will diagnose which components are at fault in either case.
My ATAG boiler is still under warranty — do I need to pay for this repair?
ATAG offers a 10-year manufacturer warranty on many of their boilers, and during this period they send their own engineers to diagnose and fix faults at no cost to you (subject to warranty terms and conditions). Before booking and paying for an independent Gas Safe engineer, call ATAG directly on 0800 680 0100 to check whether your boiler is still covered. Have your boiler model and installation date to hand.