Overview
The C7 216 fault on a Worcester Bosch Greenstar i/System or combi boiler means the flue fan is not running at the expected speed during start-up (either not running at all or running too slowly). The fan is essential for creating the correct airflow to expel combustion gases and allow safe ignition; when the control board detects incorrect fan speed it locks the boiler out as a safety measure to prevent unsafe combustion and possible flue gas or carbon monoxide buildup. Common causes are a seized or failing fan motor, a blocked or restricted flue, damaged or loose wiring between the fan and the PCB (fan harness/connector), or a fault on the control board itself. Severity is moderate to high because the boiler will not run and continued attempts to use it with a compromised flue/venting arrangement could create a hazardous situation. Basic, non-intrusive checks (reset, listen for the fan, check external flue for obvious blockages, check system pressure) are reasonable for a homeowner, but diagnosis and any internal electrical or mechanical repairs should be done by a qualified Gas Safe heating engineer.
Possible Cause: Fan speed too low.
Troubleshooting Steps
Safety first: if you smell gas, hear a gas leak, or a carbon monoxide alarm has sounded, turn off the gas supply and electricity if it is safe to do so, ventilate the property, leave immediately and call the gas emergency service or emergency services. Do not attempt internal electrical or sealed-compartment work unless you are a qualified Gas Safe engineer. Never test live voltages or dismantle gas-carrying components if you are not trained.
Initial checks a homeowner can safely do:
1) Record the exact error code and any flashing lights, then try a basic reset. You can reset many Worcester Greenstar i boilers by pressing and holding the reset or ✓ button for about 8 seconds, or isolate the boiler at the electrical switch for 30 seconds and turn it back on. If the fault does not reappear after a reset and the boiler runs normally, monitor it; if it returns, continue with the checks below.
2) Listen during ignition/start-up for fan operation. If you hear nothing or only a faint sound, the fan may be seized or not receiving power.
3) Check the external flue terminal (outside) for obvious blockages: bird nests, leaves, buildup, or physical damage. Remove external obstructions if safe to reach; do not force or probe the flue.
4) Check boiler pressure: the system should normally be around 1–1.5 bar when cold. Low pressure can cause faults; top up using the filling loop if you know how and it is safe to do so. If pressure keeps dropping, call an engineer.
Specific diagnostic and safe inspection steps:
5) Visually inspect the accessible fan wiring and connector only if the boiler casing can be opened easily without breaking seals and only after isolating the mains power. Look for loose connectors, disconnected plugs, burnt or melted plastic, corrosion, or broken wires. Do not touch exposed terminals and do not remove sealed covers. If you see damaged wiring or a loose connector, turn the boiler off and call a Gas Safe engineer.
6) If you are familiar with the boiler’s user/service menu and comfortable using it, there is usually a fan test (T2) in the diagnostics which will run the fan without firing the gas. Running diagnostics can confirm whether the fan spins and how it behaves. If you are unsure, leave this to an engineer.
7) Check for signs of water ingress or debris in the fan housing or flue route (sometimes condensate or pests cause partial blockage). Do not disassemble the fan housing yourself; note findings and report them to the engineer.
8) If the fan does not start, runs slowly, or the fan connector appears damaged, or the fault returns after reset and basic checks, do not attempt to bypass safety controls. The likely next steps are replacement or repair of the fan motor, repair/replace the fan harness, or checking/replacing the PCB — all of which require a qualified engineer.
When to call a professional: Call a Gas Safe qualified engineer if the C7 fault remains after reset and the above safe checks, if you find damaged wiring or connectors, if the fan does not run or runs slowly on diagnostics, or if you detect repeated or intermittent faults. An engineer will safely isolate the appliance, perform live electrical tests, run service diagnostics, clear or repair the flue and fan assembly, and replace components (fan, harness, or PCB) as needed. Do not attempt internal electrical repairs or component replacements yourself. Keep the boiler off and tagged if an unsafe condition is suspected until repaired.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Worcester Bosch Greenstar I System / Combi Boiler.