Baxi 4 Red Flashes Fault Code: Causes, Fixes & Repair Costs
What does the Baxi 4 Red Flashes fault code mean?
Four red flashes on a Baxi boiler indicate a fan fault. The boiler's control system has detected that the fan is not performing as expected and has shut the boiler down as a safety precaution. The fan plays a critical role in two ways: it draws fresh air into the combustion chamber so gas can burn correctly, and it expels the resulting flue gases safely out of the property. Without a confirmed fan signal, the boiler will not allow gas to reach the burner — a deliberate safety interlock. The boiler will remain locked out until the underlying problem is identified and the fault is cleared. For context, Baxi uses a flash-code system across its range: 1 flash relates to a sensor or flow temperature fault; 2 flashes indicate an overheat trip; 3 flashes mean repeated ignition failures; 4 flashes point to a fan issue; 5 flashes indicate a Parameter Storage Unit (PSU) error; and 6 flashes cover miscellaneous faults.
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 failed fan motor Common
The fan motor itself is the most frequent cause of this fault code. Over time the motor windings can burn out or the internal bearings can seize or deteriorate, preventing the fan from reaching the correct speed or from spinning at all. The boiler's PCB monitors the fan speed signal and will lock out if no valid signal is received.
- Blocked or obstructed flue terminal Common
Debris, bird nests, leaves, or ice can partially or fully block the flue terminal on the outside wall or roof. This restricts airflow, causes back-pressure, and prevents the fan from clearing flue gases at the required rate — triggering the same safety response as a mechanical fan failure.
- Faulty air pressure switch Common
The air pressure switch monitors the pressure differential created by the fan to confirm it is running correctly before firing begins. If the switch's diaphragm fails, the internal contacts stick, or the rubber hose connecting it to the fan housing cracks or becomes loose, the boiler receives no confirmation that airflow is adequate and locks out.
- Fan wiring or electrical connection fault Sometimes
The wiring harness between the fan and the PCB can suffer from loose connectors, corrosion, or heat-related damage. A break or high-resistance connection in this circuit can prevent the fan speed signal from reaching the control board, causing a spurious fan fault even when the motor itself is still functional.
- Frozen condensate pipe Sometimes
In cold weather the external section of the condensate pipe can freeze solid. The resulting blockage causes flue gas and pressure fluctuations inside the boiler that mimic — or compound — a fan or airflow fault. This is a seasonal but very common winter cause of lockout on all condensing boilers, including Baxi models.
- PCB fault Rare
The PCB processes the tachometer signal from the fan and decides whether to allow ignition. If the relevant section of the PCB is damaged or develops a fault, it may fail to recognise a perfectly healthy fan signal and raise a spurious 4-flash lockout. PCB faults tend to be a diagnosis of exclusion — confirmed only after the fan, wiring, and pressure switch have been checked.
How to fix it
- Reset the boiler once DIY safe
Press and hold the reset button (usually marked with a flame or reset symbol on the boiler fascia) until the red flashes clear and the boiler attempts to restart. Allow the boiler a full start-up cycle before judging whether the fault has cleared. Do not reset more than two or three times in total — repeated resets without a successful start indicate a genuine fault that needs professional diagnosis and can mask developing problems.
- Check your gas supply is working DIY safe
Confirm that other gas appliances in your home (hob, oven, gas fire) are operating normally. If none of them work, the issue is with your gas supply rather than the boiler — contact your gas supplier or call the National Gas Emergency number (0800 111 999) if you suspect a leak.
- Inspect the flue terminal and external vents for blockages DIY safe
From outside the property, visually check the flue terminal (the white plastic fitting on the wall or roof where exhaust exits). Look for obvious obstructions such as bird nests, leaves, debris, or ice. If the terminal is safely accessible at ground level, clear any loose material carefully. Never attempt to access a flue on a roof without proper access equipment, and do not poke anything into the flue pipe itself.
- Thaw a frozen condensate pipe if cold weather is the likely cause DIY safe
If temperatures have been below freezing and the condensate pipe runs outside or through an unheated space, it may have frozen. The pipe is typically a white or grey plastic pipe (21.5 mm or 32 mm diameter) running from the bottom of the boiler to an external drain. Gently warm the frozen section using warm — not boiling — water poured from a watering can, or wrap it in a warm (not hot) cloth. Once thawed, reset the boiler. Afterwards, insulate the exposed section with pipe lagging to reduce the risk of a repeat freeze.
- Have a Gas Safe engineer inspect and test the fan Gas Safe engineer
A registered engineer will use specialist test equipment to measure the fan motor's electrical resistance, check the tachometer output signal, and run the fan in isolation. If the motor windings are burnt out or the bearings have failed, the fan assembly will need to be replaced. Fan replacement on Baxi boilers is a straightforward job for an experienced engineer and is often completed in a single visit.
- Have the air pressure switch and connecting hose inspected Gas Safe engineer
The engineer will check the rubber pressure hose for cracks, blockages, or loose connections, and test the switch itself for correct operation. A faulty hose is inexpensive to replace; a failed switch is also a relatively low-cost repair and is often carried out at the same time as a fan replacement to rule out multiple concurrent faults.
- Have the fan wiring harness and connections checked Gas Safe engineer
The engineer will inspect the wiring connectors at the fan and PCB ends for signs of corrosion, heat damage, or loose terminals. Damaged wiring can often be repaired or the connector replaced without replacing the entire fan or PCB, making this a cost-effective early check.
- If all other components test correctly, have the PCB assessed Gas Safe engineer
Should the fan, pressure switch, wiring, and flue all test satisfactory, the PCB may be misinterpreting the fan signal. PCB diagnosis and replacement must be carried out by a Gas Safe registered engineer. This is typically a diagnosis of last resort given the cost involved.
Parts you may need
- Baxi fan assembly (motor and housing) · from £175
- Air pressure switch · from £35
- Pressure switch connecting hose · from £12
- Fan wiring harness / connector · from £20
- 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 £220–£420, depending on the underlying cause.
Frequently asked questions
Can I fix Baxi 4 red flashes myself?
The safe DIY steps are limited to resetting the boiler once or twice, checking the flue terminal is clear of obvious external blockages, and thawing a frozen condensate pipe in cold weather. Everything beyond that — testing the fan motor, replacing the fan, checking the air pressure switch, or inspecting the PCB — must be carried out by a Gas Safe registered engineer. Working on the fan, gas components, or control board without registration is illegal and dangerous.
How much does it cost to replace a fan on a Baxi boiler?
For most Baxi models, expect to pay roughly £220–£420 all-in for a fan replacement, which covers the part (typically £100–£200 depending on the model) and one to two hours of labour. If the air pressure switch also needs replacing, add around £35–£60 for the part. In the less common scenario where the PCB also requires replacement, costs can rise to £500 or more — but this is unusual and an engineer should rule out simpler causes first.
Why does my Baxi boiler keep showing 4 red flashes after a reset?
If the fault returns within a short time of resetting, it means the underlying problem has not been resolved. The most likely explanation is that the fan motor has mechanically failed, the flue has a persistent blockage, or the air pressure switch has failed. Stop resetting and arrange for a Gas Safe engineer to inspect the boiler — repeated lockouts without investigation can mask a worsening fault and put unnecessary strain on other components.
Could 4 red flashes be caused by a frozen condensate pipe?
Yes — particularly in cold snaps. A frozen condensate pipe creates a blockage that causes pressure fluctuations inside the boiler, which can trigger airflow-related fault codes including 4 red flashes. If your boiler has gone into lockout overnight during freezing temperatures and you can identify a frozen section of the condensate pipe outside, thawing it with warm water and then resetting the boiler will often clear the fault. Insulate the pipe afterwards to prevent recurrence.