ATAG 151 Fault Code: Causes, Fixes & Repair Costs
What does the ATAG 151 fault code mean?
Fault code 151 on an Atag boiler indicates that the fan speed has fallen outside the acceptable tolerance for at least 25 seconds, or that the PCB has lost communication with the fan via PWM (Pulse Width Modulation). In simple terms, the boiler's control board is either not receiving a valid speed signal from the flue fan, or the fan itself is not spinning at the expected rate. Because the fan is essential for safe combustion and flue gas extraction, the boiler locks out rather than attempt to fire. On Atag iR-series models there is an additional twist: a flow switch monitors water movement through the heat exchanger during the ignition sequence, and if water flow is insufficient this switch interrupts the fan circuit — causing the boiler to display 151 as a fan error even though the underlying problem is actually a lack of water circulation. This can make diagnosis less straightforward than the fault code alone suggests.
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
- PCB not communicating with the fan (PWM failure) Common
Atag's own diagnostic guidance notes that the PCB is at fault in a high proportion of 151 errors. The control board sends a PWM signal to regulate fan speed; if that signal is absent or corrupted, the fan either runs uncontrolled or not at all, and the boiler locks out. This can be confirmed by disconnecting the PWM connector at the fan — if the fan then runs freely at full speed, the PCB's PWM output is the culprit rather than the fan motor itself.
- Faulty or failed flue fan Common
The fan motor itself can wear out, seize, or develop a fault in its internal tachometer (speed-sensing) circuit. If the PWM signal from the PCB is proven healthy but the fan still does not run correctly, the fan assembly requires replacement. Note that if a failing fan has drawn excessive current it may also damage the PCB, meaning both components need replacing at the same time.
- Wiring harness or connector fault Sometimes
Corroded, loose, or damaged wiring between the PCB and the fan — particularly the 4-wire PWM connector — can interrupt the speed signal without either component being inherently faulty. Engineers check continuity through the harness as a standard step before condemning the fan or board.
- Insufficient water flow triggering the flow switch (iR models) Sometimes
Atag iR boilers include a flow switch that checks for adequate water movement through the heat exchanger before allowing ignition to proceed. If system pressure is low, the pump is weak, thermostatic radiator valves (TRVs) have closed off too many radiators, or there is no bypass fitted, the flow switch interrupts the fan circuit and produces a 151 fault. This is a secondary cause that the boiler's error code does not make immediately obvious.
- Pump fault causing inadequate circulation Rare
Because the pump on Atag boilers is controlled directly by the PCB, a failing pump that cannot generate sufficient flow will prevent the flow switch from closing, leading to the same fan interruption described above. Conversely, a pump running continuously when it should not may indicate a PCB logic fault.
How to fix it
- Check the basics before anything else DIY safe
Confirm that your gas supply is working (try another gas appliance such as a hob), that the boiler's system pressure is between 1 and 1.5 bar on the pressure gauge, and that no other appliances or circuit breakers have tripped. These checks cost nothing and occasionally reveal a separate issue that mimics a fan fault.
- Top up system pressure if it is below 1 bar DIY safe
If the pressure gauge reads below 1 bar, use the filling loop (a braided or rigid pipe usually beneath the boiler with one or two valves) to slowly add water until the gauge reads around 1.2 bar, then close the valve(s). Low pressure can reduce pump flow enough to trip the flow switch on iR models, producing a 151 code.
- Attempt a boiler reset DIY safe
Press and hold the reset button (or follow the reset procedure in your user manual) to clear the lockout. If the boiler fires and runs normally, monitor it over the following days. Some users report that an occasional 151 error clears permanently after a service, suggesting a minor communication glitch. Do not reset more than two or three times — repeated lockouts mean the underlying fault needs professional attention.
- Check that TRVs are not closing off all radiators simultaneously DIY safe
If thermostatic radiator valves have been fitted throughout the house and all are turned down at once, the system has no path for water to circulate. Ensure at least one radiator is left without a TRV, or that the TRV on one radiator is fully open, so the pump always has somewhere to push water. This is a homeowner-safe adjustment that can resolve intermittent 151 faults on iR models.
- Call a Gas Safe registered engineer to carry out PWM and fan diagnostics Gas Safe engineer
An engineer will safely isolate the boiler electrically and disconnect the PWM connector at the fan. With power restored, the fan should run at full speed if the fan motor is healthy. If it does not run, the engineer checks for 230 V at PCB connector X11 pins 1 and 2. No voltage points to PCB replacement; 230 V present with a non-running fan points to the wiring harness or the fan itself. This diagnostic sequence, set out in Atag's own fault-finding documentation, pinpoints the faulty component before any parts are ordered.
- Engineer to inspect the flow switch and pump (iR models) Gas Safe engineer
If the fan and PCB both test as healthy, the engineer will check the flow switch for correct operation and assess pump output. Increasing pump speed by one setting has resolved intermittent 151 faults in some cases. A blocked or sticking flow switch may need cleaning or replacement.
- Engineer to replace the fan, PCB, or both as indicated Gas Safe engineer
Once the diagnostic steps have identified the faulty component, the engineer will fit a replacement. If a failing fan has damaged the PCB through overcurrent, both parts will need to be replaced together. Always ask the engineer to confirm whether the boiler is still within Atag's warranty period (standard 10 years, extendable to 14 years with annual servicing) before authorising paid repairs — contact Atag on 0800 680 0100 if unsure.
Parts you may need
- Atag flue fan assembly · from £150
- PWM wiring harness / fan connector loom · from £35
- Atag PCB (main control board) · from £280
- Flow switch (iR models) · from £45
- Circulation pump · from £95
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 £250–£500, depending on the underlying cause.
Frequently asked questions
Can I fix Atag fault code 151 myself by just resetting the boiler?
A reset is always worth trying once or twice — some isolated 151 errors are caused by a brief communication glitch and clear permanently after a reset or a service. However, if the fault returns after resetting, or if it appears repeatedly over a short period, a Gas Safe registered engineer needs to carry out the PWM and fan diagnostics described above. Repeatedly resetting a boiler that has a genuine fan or PCB fault will not fix it and may mask a developing problem.
Why does my Atag iR boiler show a fan fault when the fan looks fine?
Atag iR models include a flow switch that monitors water circulation through the heat exchanger. If water flow is insufficient during the ignition sequence — due to low system pressure, a weak pump, or all TRVs being closed simultaneously — the flow switch interrupts the fan circuit and the boiler displays fault code 151 even though the fan itself is perfectly healthy. Checking system pressure and ensuring at least one radiator is always open are the first steps for iR owners.
How much does it cost to fix an Atag 151 fault in the UK?
Most homeowners pay between £250 and £500 all-in for parts and labour. A fan replacement typically costs £200–£400 including fitting, while PCB repair (minor connector or solder work) usually falls in the £250–£400 range. A full PCB replacement is more expensive, typically £450–£700 including labour and VAT, and this sits above the headline repair range. Prices tend to be higher in London and the South East. If your boiler is within Atag's 10-year warranty period, the repair may be covered at no cost — call Atag on 0800 680 0100 before authorising any chargeable work.
Does fault code 151 always mean the PCB needs replacing?
Not necessarily, though Atag's own documentation acknowledges that the PCB is at fault in a significant proportion of cases. A Gas Safe engineer should work through the diagnostic steps first: disconnecting the PWM connector to see whether the fan runs freely, checking for 230 V at the relevant PCB terminals, and testing the wiring harness for continuity. Sometimes the fault is simply a loose or corroded connector, which is a much cheaper fix. Only replace the PCB once the diagnostics have pointed clearly in that direction, and check whether a fan failure has also damaged the board before ordering parts.