Overview
D.4 on a Vaillant EcoTEC Plus is a diagnostic code indicating a problem with the heating flow temperature sensor (the flow NTC thermistor) or its wiring/connection. In plain terms the boiler is not getting a valid temperature reading from the flow sensor so the control electronics cannot correctly regulate or protect the heating circuit. Common causes are a disconnected or loose plug, a broken or shorted cable, a faulty thermistor, or an incorrect plug connection on the PCB or sensor harness. Severity: this is a safety-related sensor fault. The boiler will often go into a restricted mode or lock out to protect itself (it may refuse to run the central heating or shut down automatically). It is not normally an emergency in the sense of an immediate gas leak, but you should treat it seriously because the boiler’s temperature regulation and safety cut-outs rely on this sensor. A temporary reset may clear it once, but persistent or repeating D.4 faults require professional attention. DIY vs professional: you can do some safe, simple checks (reset the boiler, check visible external connectors, check system water pressure and obvious damage). However, diagnosing and repairing the sensor, wiring harness, plugs or replacing components inside the boiler requires a qualified Gas Safe engineer. Do not attempt internal electrical or gas work yourself.
Possible Cause: Current temperature for warm stat sensor
Troubleshooting Steps
Safety precautions: Before doing anything, switch the boiler to standby or OFF, isolate electrical power at the fused spur if accessible, and if you open any external covers only do so if you are confident and not exposing internal live components. Do not attempt to repair or replace internal components, wiring, or gas parts yourself. If you smell gas, evacuate and call your gas emergency number immediately.
Initial checks a homeowner can do safely:
1) Reset the boiler: try a simple reset via the boiler controls and allow it to restart. If the D.4 reappears immediately or after a short time, further action is needed.
2) Check system pressure: look at the pressure gauge on the front of the boiler. If pressure is very low (well below ~1 bar) some faults can occur — top up only using the correct filling loop procedure for your system if you know how; if unsure call your installer.
3) Inspect for obvious external damage or loose external plugs: without removing internal panels, look for any obvious loose cable runs, water leaks, or corrosion around the boiler and nearby pipework. Note any recent work on the heating system or electrical work which might have disturbed wiring.
4) Note accompanying codes/behaviour: record exact error codes, when they occur (on heat call, at startup), and take photos — this helps the engineer.
Specific diagnostic steps and likely fixes (professional work required):
1) Diagnostic readout: a trained engineer will check the diagnostic display values for the flow sensor (the NTC resistance/temperature reading in the d-menu) to confirm if the reading is open-circuit, short-circuit or implausible vs actual water temperature.
2) Check plugs and harness: the common causes are unplugged or loose NTC plug or incorrect plugs on the PCB. The engineer will isolate power, open the boiler and inspect the sensor plug(s), harness connectors on the PCB and actuatorSTOR connections if applicable. Re-seating connectors can sometimes cure the fault if a plug is loose.
3) Inspect wiring for damage: the wiring harness will be inspected for chafing, pinched cables, shorts or water damage. If the cable is broken or shorted it will need replacement.
4) Test or replace the NTC thermistor: using a multimeter the engineer will measure the NTC resistance at ambient and/or with the system cold/hot to verify the sensor is within expected values. If defective, the flow NTC must be replaced.
5) Check for related faults: if D.4 appears alongside other fault codes (for example F.91, F.20 or pressure/flow faults) the engineer will investigate the full system (pump operation, flow-switch/mass-flow sensor, pressure transducer) to ensure there is not a wider issue causing implausible readings.
6) PCB or electronics: if wiring and sensor are good but the control board reads implausible values the main PCB or connector circuitry may be faulty; testing and replacement of PCB components require a qualified engineer.
When to call a professional: If the code persists after a reset, if you find any internal wiring damage, or if any work beyond the simple visual checks is needed, stop and call a Gas Safe registered heating engineer or Vaillant service. The replacement of NTC sensors, wiring harnesses, plugs or PCB components must be performed by a qualified engineer for safety, correct diagnosis and warranty reasons.
What to tell the engineer: give them the exact model and serial number of the boiler, the D.4 fault code and any other F-codes shown, detail when the fault occurs, and what checks you have already done (reset, pressure, photos). This speeds up diagnosis.
Do not include price estimates or try to do internal electrical/gas repairs yourself. A qualified Gas Safe engineer should carry out the final repair and testing.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Vaillant EcoTEC Plus Regular Gas Boiler.