Error E11

Safety first

1) If you attempt any checks: isolate the boiler electrical supply at the fused spur or consumer unit before removing the boiler cover. Allow the boiler to cool. If you are not confident or competent with electrical isolation, do not remove the cover and call an engineer. Do not tamper with the gas supply or gas valve. Any work involving gas or control PCB replacement should be performed by a Gas Safe registered engineer.

Initial homeowner checks (no tools or only very simple tools)

1) Note any other fault codes displayed; multiple codes can point to a control box or wider electrical issue. 2) Try a basic reset using the boiler reset button or switching the boiler off and on at the isolator. If E11 clears and does not return, monitor for recurrence. 3) Check system pressure on the pressure gauge; very low pressure can cause circulation problems and odd sensor behaviour. 4) With heating called for, listen for the circulating pump and feel the outer flow and return pipes (careful — they may be hot). The flow pipe should be noticeably hotter than the return when the boiler is running.

Checks you can do if comfortable and able to isolate power (only proceed if you understand electrical isolation)

1) Isolate mains power, remove the outer case per the manual, and visually inspect the flow and return thermistors and their connectors. The thermistors are small sensors clipped to the flow and return pipes. 2) Check the connectors are pushed fully onto their terminals and that wires are not corroded, broken, or crushed. Reseat push-on connectors. 3) Verify each sensor is clipped to the correct pipe: flow sensor on the flow/outlet pipe from the heat exchanger, return sensor on the return/inlet pipe. If sensors are swapped the boiler can fail calibration. 4) Look for damaged insulation, melted cable, or signs of water ingress around connectors or the PCB.

Basic diagnostic measurements for competent DIYers only

1) With the power isolated and if you have a multimeter, you can check continuity/resistance of each thermistor lead according to the service manual values (if available). If a sensor reads open circuit or a short, replace the thermistor. If you are unsure of correct resistance values do not guess; call an engineer. 2) With heating running, compare pipe temperatures with an infrared thermometer or surface thermometer. Both sensors should show different temperatures in normal operation (flow hotter than return). Large or no difference may indicate circulation issues or sensor faults.

Simple fixes you can try

1) Turn off power, reseat the connectors and ensure sensors are clipped correctly to the pipe with good metal contact (remove old tape or debris). 2) If a sensor is visibly damaged, replace the thermistor assembly only if you are competent and have isolated power; follow the manual for replacement steps and reassemble carefully. 3) After reassembly, power the boiler and test. If E11 clears and stays clear, monitor the system.

When to call a professional

1) If the fault persists after the above checks, if you find no obvious connector/sensor problem, or if you discover wiring damage, you should call a Gas Safe registered engineer. 2) If the fault is intermittent or accompanied by other electronic fault codes (E10, E12-E22, E31-E37 etc), the control box (PCB) may be at fault and will require diagnostic equipment and replacement by a qualified engineer. 3) Do not attempt to replace or repair the PCB, gas valve, or internal gas components yourself.

Final notes

1) Document what you tried and any observations (which sensor looked damaged, whether reseating connectors cleared the fault) before the engineer arrives. 2) For safety and warranty reasons, any internal repairs, part replacements and commissioning should be completed by a qualified heating engineer. Do not operate the boiler if it displays safety-related fault codes and you are unsure of the cause.