Error D4 271

Safety precautions:

1) If you smell gas or suspect a gas leak, get everyone out, do not operate electrical switches, and call the gas emergency number immediately. 2) If you have any sign of carbon monoxide (headaches, nausea, unusual soot, pilot light issues), turn the appliance off, ventilate the area and contact a Gas Safe engineer immediately. 3) If the boiler is locked out or showing fault lights, you may perform a single reset only—do not repeatedly reset a persistent fault. 4) Do not remove the boiler casing, access burners, gas components, or internal wiring unless you are a qualified Gas Safe engineer.

Initial homeowner checks you can safely do:

1) Record the error: take a photo of the display, note exactly when it happened (heating on/DHW on/first fire up) and whether the fault is constant or intermittent. 2) Perform one reset: follow the boiler manual/reset button. If the fault returns immediately or repeatedly, stop and proceed to call an engineer. 3) Check system pressure on the boiler gauge—pressure should normally be around 1–1.5 bar when cold. If pressure is very low, repressurise following the manual and then observe. 4) Look for visible leaks in pipework or around the boiler that could cause low flow. 5) Bleed radiators to remove trapped air (airlocks can impede flow). 6) Listen at the boiler: can you hear the pump running or any unusual noises? If there is no pump hum when there should be flow, that is important information for the engineer. 7) If you have a non-contact infrared thermometer you can carefully compare the temperature of the flow pipe close to the boiler and other flow/return pipes—but do not touch hot surfaces.

Further diagnostic steps and fixes (for engineers / what to expect from a pro):

1) If homeowner checks do not clear the fault, call a Gas Safe registered Worcester Bosch-trained engineer and give them the recorded fault details and photos. 2) The engineer will place the boiler in service mode and check real-time temperatures and sensor readings, plus diagnostic logs. They will measure the resistance (ohms) of the flow NTC and the safety sensor / safety temperature limiter at ambient and operating temperatures to verify correct NTC behaviour. 3) They will inspect wiring harnesses and connectors for corrosion, loose connections, chafing, short circuits or water ingress—particularly around sensor plugs. 4) The safety temperature limiter (safety sensor/LIM) will be continuity-tested; a failed limiter or a shorted/open NTC will be replaced. 5) Pump operation will be checked—electrical supply, rotation, impeller seizure, or incorrect pump speed. If the pump is not circulating properly it will be repaired or replaced. 6) The engineer will check for blockages, airlocks or partial restriction in the plate heat exchanger, diverter valve or system pipework and may perform a system flush if scaling or blockage is present. 7) If sensors and wiring are good but temperatures still mismatch, the control PCB will be assessed; a control board replacement may be necessary if it is misreading or providing incorrect reference signals. 8) After repairs/replacements the engineer will run the boiler through several cycles, verify stable flow and safety temperatures, and clear stored fault codes.

Important notes:

1) A single manual reset is acceptable for an isolated transient fault. Repeated lockouts or a persistent D4 271 should not be ignored—turn the boiler off and get a Gas Safe engineer to diagnose. 2) Do not bypass or attempt to reset safety limiters or bypass sensors; these are life-safety devices. 3) When you call an engineer, provide the model, the exact fault code (D4 271), how often it occurs and the conditions (heating on/first fire up/cold start) and any photos—this speeds diagnosis. 4) If the boiler is under warranty or part of a service plan, mention that when you book the engineer.

Summary: try a single safe reset, check pressure, bleed radiators and look for obvious issues. If the fault comes back, or if you have no pump noise or very hot pipes, switch the boiler off and call a Gas Safe registered engineer to test sensors, wiring, pump and the control system. Do not attempt internal repairs or sensor replacements yourself.