Error S.16

Safety precautions:

1) If the boiler is showing a fault, switch it off at the room thermostat and the boiler’s on/off control; if instructed in the manual, you may isolate the boiler from the mains supply before doing any visual checks. Do not open the boiler casing, attempt gas adjustments, or work on internal wiring unless you are a qualified Gas Safe engineer. Ensure good ventilation around the boiler and keep children/pets away.

2) If you smell gas, evacuate the property, do not operate electrical switches, and call your gas emergency number immediately.

Initial homeowner checks (non-intrusive):

1) Record the exact code(s) shown and the sequence of events (does S.16 appear only when you ask for hot water, or during heating, and are there any other F.xxx codes displayed?). This information is useful to the engineer.

2) Try a simple reset: switch the boiler off, wait 1–2 minutes, then switch it back on and ask for hot water again. Note if the fault returns immediately or after some time.

3) Check the air intake and flue terminals outside the building for visible blockages (bird nests, leaves, debris, snow). A blocked flue/intake can affect fan and combustion performance. Clear only obvious external obstructions safely from the outside.

4) Check condensate pipe (if weather cold) to ensure it is not frozen or blocked. A blocked condensate can cause a range of faults. Thaw frozen condensate with warm (not boiling) water; if in doubt call an engineer.

5) Check visible wiring and plug connections outside the boiler (for example remote controls or external cylinders) for obvious damage. Do not remove the boiler casing to check internal plugs.

6) Check system water pressure on the boiler gauge; while not specific to S.16, low pressure can cause other faults and make diagnosis harder.

Specific diagnostic steps for a competent homeowner (only visual and external):

1) Note if S.16 is accompanied by fan-specific faults such as F.32 or sensor/NTC faults like F.10 / F.13 / F.91. If multiple codes appear, list them for the engineer.

2) If comfortable and safe, switch the boiler off at the isolator and remove the front cover only if the manual says it is permitted for user checks and the cover is not sealed. If the cover is sealed or instructions prohibit removal, do not remove it. If you do remove a user-serviceable panel, look for loose connectors at the fan plug or obvious burn marks—do not touch internal components.

3) If you can access the fan area without voiding warranty or safety seals and you are competent to do so, look for foreign objects or debris preventing impeller rotation. Do not power the fan by hand, and do not operate the boiler with the casing off.

4) If the fault only happens in DHW mode, check the DHW cylinder sensor and connections if your system has an external cylinder. A faulty NTC / cylinder thermostat or its wiring can present alongside fan-related status codes; ensure its plug is seated and cable isn’t visibly damaged.

When to call a professional and what they will do:

1) Call a Gas Safe registered engineer if the fault persists after the basic checks and reset, or immediately if you are unsure. Do not attempt to replace the fan, PCB or internal sensors yourself. These parts are linked to combustion safety and require specialist tests and calibrated replacements.

2) The engineer will run diagnostic tests: read and clear fault history, check for correlated fault codes (fan hall sensor, flow/return NTCs, eBUS communications), measure fan supply and hall sensor signals, test NTC resistances and wiring, check the STB/flue temperature limiter, and inspect the main PCB and connectors.

3) Typical professional fixes include reseating or replacing faulty connectors, replacing a defective fan motor or fan hall sensor, replacing faulty NTC thermistors or their wiring/plugs, cleaning/clearing the flue/airway, or replacing the main PCB if electronics are at fault. If the engineer finds blocked or sooted parts they will clean and retest safety devices.

4) Make sure the engineer explains the fault cause, what was repaired or replaced, and any recommended preventative steps (e.g., annual service, clearing flue/airways, system cleanliness).

Summary note: S.16 indicates a fan/ventilation-related overrun or sensor issue during hot water mode. Try the simple external checks and a reset but do not open the boiler or attempt internal electrical or gas repairs — call a Gas Safe qualified engineer to diagnose and fix fan, sensor, wiring, or PCB faults safely.