Boiler
E125
The E125 error code on a Baxi boiler indicates a fan speed or air pressure fault — it’s a lockout code, meaning your boiler has shut itself down as a safety precaution and won’t restart until the issue is resolved.
What does this error mean?
The E125 code tells you the boiler’s control board has detected that the fan isn’t running at the correct speed, or that the air pressure switch isn’t confirming adequate airflow through the flue. Without correct airflow, combustion gases can’t be safely expelled, so the boiler locks out to prevent a dangerous build-up. This is a safety-critical fault and should be taken seriously.
Est. repair cost
£100–£400 depending on the faulty component (pressure switch or fan); diagnosis call-out typically £80–£120
🔍 Likely causes
🛠️ Step-by-step fix
Step 1 — Reset the boiler: Press and hold the reset button (usually marked with a flame symbol) for 3–5 seconds. If the boiler fires up and runs without faulting again, monitor it closely over the next 24 hours.
Step 2 — Check the flue externally: Go outside and visually inspect the flue terminal on the wall or roof. Make sure it’s clear of debris, ice, or obstruction. Do not attempt to reach or dismantle the flue yourself — just check for obvious blockages you can see.
Step 3 — Check the condensate pipe: In cold weather, the condensate pipe (the white plastic pipe that exits through an outside wall) can freeze. Thaw it using warm (not boiling) water poured over the external section, then reset the boiler.
Step 4 — Do not attempt any further DIY: The fan, air pressure switch, hoses, and PCB are all internal gas appliance components. Working on these without Gas Safe registration is illegal under UK law and potentially dangerous. Stop here and call an engineer.
Step 5 — If the boiler re-locks immediately after reset, do not keep resetting it repeatedly. Multiple resets without a fix can mask the fault and delay proper diagnosis.
📞 When to call a professional
If the boiler re-locks after a single reset, or if there is no visible flue obstruction or frozen condensate pipe, call a Gas Safe registered engineer immediately — do not attempt to investigate internal components yourself. Tell the engineer the exact error code (E125) and describe how often it’s been locking out and whether it happens during start-up or mid-cycle, as this helps them diagnose the fault faster.
⚖️ Repair or replace?
A fan replacement typically costs £200–£400 all-in with parts and labour; an air pressure switch is cheaper at around £100–£200. If your Baxi boiler is under 10 years old and otherwise reliable, repair is almost always the better option. If it’s over 12–15 years old and this isn’t the first fault, a new A-rated boiler (typically £1,800–£3,000 installed) is worth serious consideration for efficiency savings and reliability.