Error 166

Safety first

1. If you smell gas, see soot, or suspect a carbon monoxide leak: stop using the appliance, ventilate the area, leave the property and call the gas emergency number and a Gas Safe engineer immediately. Do not attempt further checks.

2. Before opening the boiler casing for inspection, isolate the electrical supply at the fuse box and, if you know how, turn off the gas supply at the meter. If you are not comfortable doing this, do not open the boiler — call an engineer.

3. Wear eye protection and gloves if you handle external fittings. Do not attempt to adjust or replace gas components or internal wiring unless you are a qualified, Gas Safe registered engineer.

Initial homeowner checks (safe, non-invasive)

1. Note the displayed error and the boiler model and serial number. Take a photo of the error code and any LEDs — this helps a visiting engineer.

2. Try a single reset of the boiler. Consult the boiler manual for the reset procedure (typically a reset button or turning the selector to reset and holding for a few seconds). If the boiler restarts normally and stays running, monitor for recurrence. If it locks out again quickly, do not repeatedly reset — you need professional help.

3. Check the external flue terminal for obvious blockages (leaves, bird nest, debris, ice) and clear any safe-to-remove obstruction. Do not insert objects into the flue; only remove loose external debris. In cold weather, check for a frozen condensate pipe and thaw it with warm (not boiling) water if accessible.

4. Verify the gas supply to the property: check other gas appliances (hob) are working. If the supply is off, contact your gas supplier. If gas supply is fine, continue checks.

Basic visual inspection (only if you are competent and have isolated power and gas)

1. Switch off mains electricity to the boiler and, if you know how, turn the gas supply off. Remove the outer casing following the manufacturer’s instructions. If you are not confident, stop and call an engineer.

2. Visually inspect the small vacuum hose(s) that run from the fan/ducting to the air pressure switch. Look for disconnection, kinks, splits, or blockages. If the hose is disconnected and you are certain it fits, reconnect it securely. If it is cracked or perished, it will likely need replacing by an engineer.

3. Check that the air pressure switch connector wiring is secure (visual inspection only). Do not attempt to rewire or measure voltages unless you are qualified.

4. Inspect the fan for obvious jamming or heavy soot build-up. If the fan appears seized or heavily contaminated, do not attempt to strip it — this requires an engineer.

If the problem appears to be a faulty pressure hose, loose connection, or external flue debris and you have securely reconnected or cleared the blockage, reassemble the casing, restore power/gas and try one reset. If the boiler starts and runs normally, monitor it closely for recurrence.

When to call a professional

1. Call a Gas Safe registered engineer if the error repeats after the above basic checks, if you found damaged hoses or internal components, if the fan appears faulty, if there is any sign of combustion problems (soot, smell), or if you are not confident performing the visual checks. The engineer can test the pressure switch, fan, and PCB safely and replace components as required.

2. Do not attempt to replace the air pressure switch or perform internal gas/electrical repairs unless you are a qualified engineer. Replacement and calibration of safety devices and working on gas appliances must be done by someone Gas Safe registered.

What information to give the engineer

1. Provide the boiler make, model and serial number, the exact error code (166), a description of any steps you have already taken (reset, cleared flue, checked hose), and photos you took. This speeds diagnosis and helps the engineer bring correct parts.

Final note

Air pressure switch faults are commonly resolved by cleaning/clearing the flue/airways, replacing a perished hose, or replacing the switch or fan. However because the issue involves combustion safety, prompt attention by a qualified engineer is the correct and safe course if basic external checks do not clear the fault.