Error 4 flashes 5 second gap 1 long flash

Safety precautions:

1) If you smell gas, do not operate the boiler. Evacuate the property, call your gas emergency number immediately and a Gas Safe engineer. 2) If you are unsure, or uncomfortable, stop and call a Gas Safe registered engineer. 3) Isolate electrical power to the boiler at the fused spur or mains before attempting any physical checks on the appliance (but note: many initial checks below do not require opening the boiler). 4) Do not remove any sealed covers or attempt to access live components inside the boiler unless you are a qualified engineer.

Initial homeowner checks you can safely do:

1) Record the exact fault pattern (the flashing sequence) and note any other displayed symbols. This helps the engineer. 2) Try a simple reset: use the boiler reset or switch the boiler off at the mains spur for 30 seconds and switch back on, then request heat. If the fault clears and does not return, monitor for recurrence. 3) Check mains power and timers/room thermostat: ensure the boiler has power, the programmer/stat is calling for heat and any external controls haven’t locked the boiler out. 4) Check system water pressure at the boiler gauge (typical cold pressure ~1–1.5 bar): very low pressure can cause faults. 5) Look for obvious signs around the boiler: recent condensation/drips, damp or water ingress near the boiler, frozen condensate pipe, or visible damage to external wiring or sensors (only visible external wiring; do not open the case). 6) If the boiler has recently had a power cut, note that some internal tests may run and an engineer visit could be needed if the fault persists.

Diagnostic steps an engineer will perform (do not attempt unless Gas Safe qualified):

1) Reproduce and read the fault memory with the boiler diagnostics to confirm the sensor-test failure and check for any related secondary faults. 2) Visual inspection: check sensor connectors, loom and plugs for corrosion, chafing, water ingress or disconnected leads. 3) Measure NTC sensor resistances and compare to expected values (at known ambient temperature) to confirm open- or short-circuit or incorrect thermistor characteristic. 4) Check continuity of wiring harness and look for earth/shorts to the boiler chassis. 5) If sensors test OK, check the burner control unit / HCM and wiring to the control module for faults, and run an internal sensor test routine. 6) Replace faulty sensor(s) or repair/replace damaged leads and connectors. 7) If wiring and sensors are good, the engineer will test the control electronics (burner control unit/HCM/KIM) and may replace the module if proven faulty. 8) After repairs, the engineer will clear the fault codes, run the boiler through several start/stop cycles and confirm correct flow/return temperatures and safe operation.

What you should do next:

1) If a simple reset cleared the code and it does not return, keep monitoring but arrange an engineer visit if it comes back. 2) If the fault remains after reset, or if you see signs of water or damaged wiring, contact a Gas Safe registered engineer and give them the exact fault flash sequence and any observations. 3) Do not attempt internal repairs, sensor replacements or control board work yourself. 4) If you suspect a gas leak at any time, stop, evacuate and call the emergency gas number.

Final note: "Sensor test failed" often indicates either a sensor or wiring/connector issue and is commonly resolved by replacing a faulty NTC sensor or repairing a connector, but because this affects the boiler’s safety controls you should have a Gas Safe engineer diagnose and carry out repairs. Provide the engineer with the recorded flash sequence and any steps you already tried to speed up diagnosis.