Overview
E11 on a Keston C40 / C55 combi means the boiler has detected a fault in its internal electronics — essentially a control box or PCB problem. The boiler will normally go to lockout or block mode and stop supplying heating and hot water when this fault is present. Causes can include a failed control board, bad connections between the fascia (user control panel) and the main PCB, water ingress or corrosion on the electronics, or related sensor/communication faults that the control electronics cannot reconcile. This fault is moderate-to-high severity in that the boiler will not operate normally and should not be ignored, but it is not the same as an immediate gas leak. Basic checks and a reset can sometimes clear a transient electronics error, but if the code returns it usually requires a trained Gas Safe engineer to diagnose and repair. Replacing the control box involves removing the boiler casing and working with live electrical and gas-related components, so it is not a recommended DIY job for unqualified people.
Possible Cause: Internal electronics error – rest or replace control box
Troubleshooting Steps
Safety first
- If you smell gas, evacuate the property and call the gas emergency number immediately. Do not attempt any checks.
- Do not work on internal boiler electronics, gas valves or the burner unless you are a qualified Gas Safe engineer. Isolate electrical supply before touching internal components to avoid electric shock.
- Limit resets: only try reset/power-cycle a couple of times. Repeated resets can mask an underlying fault and may cause further damage.
Initial homeowner checks (safe, no internal access required)
1) Note the details: record the exact error code (E11), how many times it has appeared, and whether it happens at a particular time (start-up, when calling for hot water, after a power cut). Take a clear photo of the display and the boiler data plate (model and serial).
2) Basic reset: press the boiler reset button as per your manual; if no dedicated reset, turn the boiler off at the mains/fused spur for 30 seconds and turn back on. Wait to see if the code reappears.
3) Check mains power: ensure the boiler is receiving power (other appliances on same circuit working, consumer unit switch not tripped). Some guides report mains voltage or frequency issues can cause board faults. If you suspect mains problems, contact a qualified electrician.
4) Visual external inspection: look for obvious signs of water leaks around the boiler casing, corrosion, burn smells or visible damage to the control panel fascia. If you find water or severe corrosion, do not switch the boiler back on and call an engineer.
If the fault remains after the basic checks
1) Do not open the boiler or attempt PCB repairs unless qualified. The next sensible step is to call a Gas Safe registered heating engineer. Provide them with: model and serial, when the fault started, number of resets attempted, and photos you took.
2) What the engineer will typically do: isolate the appliance, remove the outer case and perform electrical safety tests (mains voltage, earth continuity, polarity), check connector seating between the fascia PCB and main control PCB (the 13-way connector), inspect the control board for burnt components or water damage, check thermistor/flow and return sensor connections and continuity, and run diagnostics to see whether the error is caused by the control box itself or a wiring/sensor fault.
3) Likely fixes: if the board is confirmed faulty the control box / main PCB will normally be replaced. If the issue is a loose connector, corroded cable or a sensor wiring fault the engineer may repair or replace the affected wiring or sensor. The service will include reassembly and a full safety and combustion check before returning the boiler to service.
When to call a professional immediately
- If the E11 persists after one properly performed reset or reappears frequently.
- If you find water ingress, corrosion or a burnt smell from the boiler.
- If you are unsure about electrical supply to the boiler or suspect electrical faults.
Additional practical tips for the engineer visit
- Have the boiler model (C40/C55), serial number and any service history ready.
- Note whether any recent work, power cuts, or flooding occurred before the fault appeared.
- Do not attempt to order or fit a replacement control box yourself; replacement and commissioning should be done by a Gas Safe engineer to comply with safety and warranty requirements.
Summary
- E11 is an internal electronics/control box fault. Try one reset/power-cycle and perform safe external checks. If the fault remains or you find water damage or burnt components, call a Gas Safe engineer for diagnosis and likely control box replacement. Do not attempt internal electrical or gas repairs yourself.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Keston C40 C55 Combi Gas Boiler.