Error D4 271

Safety precautions

1. If the boiler is displaying D4 271 do not repeatedly force resets. Repeated restarts while an over-temperature condition exists can be unsafe.

2. Do not remove the boiler casing, work on gas or mains electrical connections, or attempt to replace sensors unless you are a Gas Safe registered engineer.

3. If you smell gas, switch off at the appliance isolation valve (if safe to do), evacuate the property and call the gas emergency number immediately.

4. Turn the boiler off at the programmer or main power switch before attempting any visual checks. If you are unsure, leave the boiler off and call an engineer.

Initial homeowner checks (what you can safely do)

1. Note the exact code and any other messages, the operating mode (heating or DHW) and when it happened (e.g. during a heat demand, after a restart). This helps the engineer.

2. Try a single reset using the boiler reset button or control panel. Wait to see if the fault returns. Do not perform multiple consecutive resets.

3. Check the central heating pressure on the boiler gauge. If pressure is very low (significantly below 1 bar) re-pressurise following the boiler manual, then try a single reset. Low pressure can affect pump flow.

4. Listen for the pump running during a heating demand (you should hear/feel it). Check radiator temperatures—if the flow pipe from the boiler is very hot but radiators remain cold, there may be a flow problem.

5. Bleed one or two radiators to release trapped air which can reduce flow. After bleeding, check and top up boiler pressure if it has dropped.

6. Look for obvious signs of leaks or corrosion around pipework; wet/infiltrated sensors or connectors can cause faults.

If the fault persists (and to guide the engineer)

1. Record whether the fault appears immediately on demand or only after the boiler has been running for a time. Immediate faults point more to sensor wiring/shorts or control issues; faults after running can indicate overheating due to poor flow or blockages.

2. Note any related fault codes in the boiler history if your display provides them and whether other sensor or fan codes are present.

Diagnostic and technical steps (for the engineer or to describe to the engineer)

1. Engineer to enter service mode and read live temperature values from the flow and safety sensors to confirm the differential and to see which sensor is reporting the higher/lower value.

2. Check sensor resistances (NTC thermistor values) at ambient and compare to specification to identify short/open/incorrect reading sensors.

3. Inspect sensor wiring and connector pins for corrosion, water ingress, chafing or poor connection. Repair or replace connectors/cable as needed.

4. Verify pump operation and differential pressure across the pump. A seized or failing pump, blocked filter/strainer, or closed isolation valves can cause inadequate flow and very high flow-to-safety sensor differentials.

5. Inspect the heat exchanger for scale or blockage and check the boiler flow path for blockages (including magnetic filter, inline filters, diverter valve and any system valves). If flow is restricted, the engineer may recommend cleaning or powerflushing the system.

6. Check the safety temperature limiter (safety sensor) and its correct mounting/position. If the safety sensor has tripped/failed, it may need replacing. Also confirm the flow sensor location and secure mounting.

7. Check the boiler control PCB for logged faults and test the sensor inputs on the board; if the board is faulty and misreading sensor inputs, replacement may be required.

8. After replacing any components or fixing flow issues, run the boiler through multiple call cycles to ensure temperatures and differentials are stable before leaving the appliance in service.

When to call a professional

1. If a single reset doesn’t clear the code, or if the code returns while the boiler is operating, contact a Gas Safe registered engineer. This error involves safety temperature devices and potential overheating; it should not be ignored.

2. Call an engineer if you are not comfortable with any of the safe checks above, if you detect leaks, if the pump does not run, or if there are multiple or recurring fault codes.

Notes and practical advice

1. Provide the engineer with model and serial number, the exact code (D4 271) and a description of when it occurs and any recent work or changes to the heating system.

2. If the fault is intermittent, note the time and operating conditions when it happened (cold start, high demand, after long run time). This information speeds up diagnosis.

3. Do not attempt internal electrical or gas repairs yourself. Replacement of sensors, pump work, heat exchanger cleaning or PCB replacement must be done by a qualified engineer.

Summary: perform only the safe homeowner checks listed (single reset, pressure check, bleeding radiators, visual leak check), then contact a Gas Safe registered heating engineer for full diagnostics. The likely culprits are a faulty flow or safety temperature sensor or wiring, poor flow (pump or blockage) causing overheating, or less commonly a control-board issue.