Worcester Bosch A3 Fault Code: Causes, Fixes & Repair Costs
What does the Worcester Bosch A3 fault code mean?
The A3 code is a locking fault that appears when the boiler detects a problem with its fan or the air pressure switch that monitors it. The fan plays a critical safety role: it draws fresh air into the combustion chamber and expels harmful flue gases outside. Before allowing gas to flow to the burner, the boiler checks that the fan is spinning correctly — the air pressure switch confirms this by detecting the change in air pressure the fan creates. If that confirmation never arrives, or if the fan speed is wrong, the boiler locks out and displays A3 to prevent unsafe operation. A reset is needed to clear the code once the underlying fault has been resolved, but repeated resets without fixing the root cause will not keep the boiler running.
General guidance only — not a substitute for professional advice. 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 seized fan motor Common
The fan motor is the most common culprit. Over time the motor windings can fail, the bearings can seize, or the impeller can become clogged with dust and debris. A seized fan may make a humming or grinding noise as it tries to start, or may be completely silent. Because the boiler cannot confirm airflow, it locks out immediately.
- Blocked or restricted flue terminal Common
The external flue outlet can become partially or fully obstructed by ice, leaves, bird nesting material, or even a build-up of condensate in cold weather. A blockage raises back-pressure in the flue, which the air pressure switch interprets as the fan not working correctly, triggering the A3 lockout.
- Failed air pressure switch Sometimes
The air pressure switch is a small diaphragm device that detects the pressure differential the fan creates. If the diaphragm splits, the switch contacts stick, or the thin rubber hose connecting it to the fan housing cracks or disconnects, the switch cannot confirm fan operation even when the fan itself is perfectly healthy.
- Wiring or connector fault Sometimes
Loose, corroded, or damaged wiring between the fan, the air pressure switch, and the PCB can interrupt the signals the boiler relies on to verify fan operation. Vibration over years of use can cause connectors to work loose, and damp conditions inside the boiler casing can cause corrosion at terminals.
- PCB (printed circuit board) fault Rare
The PCB controls fan speed and interprets the feedback signals from the fan tachometer and air pressure switch. If the PCB develops a fault — failed capacitors, a damaged relay, or corrupted firmware — it may incorrectly report that the fan is not running even when it is, or it may fail to command the fan to start in the first place.
How to fix it
- Check your gas supply is working DIY safe
Try a gas hob or another gas appliance. If nothing is working, the gas supply to your property may be interrupted — contact your supplier before calling a boiler engineer.
- Inspect the external flue terminal for obvious blockages DIY safe
Go outside and look at the point where the flue exits the wall or roof. Check for visible obstructions such as ice, leaves, a bird's nest, or debris. Do not poke anything into the flue terminal, but if ice is the only issue and you can safely reach it, gently pour warm (not boiling) water over the terminal to thaw it. Clear any loose external debris by hand.
- Listen carefully when the boiler attempts to fire DIY safe
Press the reset button and stand next to the boiler. A healthy fan makes a clear, consistent whirring sound in the seconds before ignition. If you hear nothing, a grinding noise, or an intermittent hum, note this down — it is valuable information for your engineer.
- Reset the boiler once DIY safe
Press and hold the reset button for around 3 seconds as described in your boiler's user manual. If the boiler fires up and runs normally, monitor it closely over the next 24 hours. If the A3 code returns — or if the boiler will not reset at all — do not keep resetting it. Repeated resets on a genuine fan fault will not fix the underlying problem.
- Call a Gas Safe registered engineer to diagnose the fault Gas Safe engineer
A qualified engineer will test the fan motor speed and tachometer output, check the air pressure switch and its connecting hose for leaks or failure, inspect all wiring and connectors in the fan circuit, and verify the flue is clear and properly sealed throughout its length. They will use a multimeter and may use a manometer to test flue pressure. Based on their findings, they will advise whether the fan, air pressure switch, wiring, or PCB needs replacing.
- Have the faulty component replaced by the engineer Gas Safe engineer
Fan motors on Worcester Bosch boilers are not user-serviceable. The engineer will source the correct OEM or compatible replacement part and fit it safely, checking system operation, flue integrity, and gas combustion readings before leaving. If the PCB is identified as the cause, discuss the repair cost versus the age and condition of the boiler before authorising the work.
Parts you may need
- Boiler fan / fan motor assembly · from £145
- Air pressure switch · from £35
- Air pressure switch connecting hose · from £12
- Fan wiring loom / connector · from £25
- 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–£420, depending on the underlying cause.
Frequently asked questions
Can I fix a Worcester Bosch A3 fault myself?
The safe DIY checks are limited to confirming your gas supply is on, clearing any visible blockage at the external flue terminal, and attempting a single reset. Everything beyond that — removing the boiler casing, testing the fan, replacing the air pressure switch, or working on the PCB — must be carried out by a Gas Safe registered engineer. Attempting internal boiler repairs yourself is dangerous, may void your warranty, and is a legal requirement in the UK for gas appliances.
How much does it cost to fix a Worcester Bosch A3 fault code?
Most people with an A3 fault end up paying somewhere between £120 and £420 depending on which component has failed. An air pressure switch is the cheapest fix — typically £100–£200 including labour. A fan motor replacement usually costs £250–£420 once parts and labour are combined. PCB replacement is the most expensive common repair, often £400–£550 or more, which is why engineers will thoroughly rule out all other causes first before recommending it. If your boiler is over 10–12 years old and facing a PCB replacement, it is worth getting a new boiler quote for comparison.
Why does my Worcester Bosch keep showing A3 after I reset it?
A reset clears the lockout temporarily but cannot fix a failed fan, a faulty air pressure switch, or a blocked flue. If the A3 code keeps returning — especially within minutes or hours of a reset — the boiler is repeatedly detecting the same underlying fault. At that point, further resets will not help and you should stop resetting and call a Gas Safe engineer to carry out a proper diagnosis.
Could a blocked flue really cause an A3 fault code?
Yes — more often than most people expect, especially in winter. When the external flue terminal becomes blocked by ice, a bird's nest, or compacted debris, back-pressure builds up in the flue system. The air pressure switch detects that the expected pressure differential from the fan is absent or incorrect and triggers the A3 lockout as a safety measure. Checking the outside of the flue terminal is always a worthwhile first step before calling an engineer.