Overview
D.28 on a Vaillant EcoTEC Plus regular boiler indicates a diagnostic fault related to the heating flow temperature sensor (the flow NTC thermistor) or its wiring, although the brief code text you supplied also mentions "switching accessory relay 2" which can point to a relay or PCB switching issue. In practice Vaillant documentation and common fault lists show D.28 symptoms as interruption or short-circuit of the flow NTC, and sometimes related cylinder/DHW NTC faults when seen alongside other codes (for example F.91). The result is the boiler cannot reliably read flow temperature, so it will lock out or go to fault to protect the appliance and prevent unsafe operation. Severity is moderate to high: the boiler may stop providing heating and/or hot water, and the underlying cause may be an electrical short, broken sensor, faulty wiring harness, or a PCB/relay fault. Because the fault involves temperature sensors, wiring and potentially the printed circuit board and gas-related controls, it is not a safe DIY repair. Basic checks and resets are reasonable for a homeowner, but any work beyond simple visual checks should be done by a Gas Safe registered engineer (or equivalent in your country).
Possible Cause: Switching accessory relay 2
Troubleshooting Steps
Safety precautions:
1. If you smell gas, evacuate the property immediately, do not operate electrical switches, and call your gas emergency number. If a CO alarm is sounding, leave the property and call emergency services.
2. Do not open the boiler casing or disconnect electrical plugs unless you are a qualified Gas Safe engineer. There are live electrical parts and gas components inside.
3. Turn off the boiler at the external switch or isolator if the appliance is behaving dangerously, but only if safe to do so.
Initial homeowner checks (safe, non-intrusive):
1. Note the exact error code(s) displayed and the boiler serial number — the engineer will need these.
2. Try a single reset: press and hold the fault-clear/reset button for one second. If the boiler attempts to restart, watch whether the code returns. Do not attempt multiple repeated resets; if it returns immediately, stop.
3. Check the boiler display and any additional fault codes (e.g. F.10/F.11/F.91) which may clarify whether the flow sensor, return sensor, or cylinder sensor is affected.
4. Check basic system items: confirm the gas isolator is open, the mains power supply to the boiler is on, and heating circuit service valves (flow and return) are open. Check the boiler pressure gauge — if pressure is very low (<1.0 bar) this can cause other faults and should be addressed first.
5. Look for obvious signs of damage or water ingress around the boiler and wiring conduits, and ensure condensate pipe is not frozen/blocked (a blocked condensate can cause unrelated faults but is worth ruling out).
6. If an external accessory or controller was recently installed or changed (a room stat, programmer, pump timer, or smart relay), try restoring it to its previous state or disconnecting that accessory according to its instructions — the D.28 text mentioning an accessory relay suggests an external device or relay could be affecting the system.
What a qualified engineer will do (diagnostic/fix steps you should expect them to follow):
1. Verify fault history and reproduce the fault if safe. Read live fault logs from the boiler controls.
2. Carry out visual checks of the PCB connectors and wiring loom without removing panels unnecessarily. Check the flow NTC plug is seated correctly and inspect for corrosion, pin damage or water ingress.
3. Measure the NTC sensor resistance with the appropriate multimeter at the sensor plug and compare to expected values at ambient temperature (the engineer will know the correct resistance chart). They will also check continuity in the wiring harness to the PCB to detect open or short circuits.
4. If an intermittent or short condition is present, isolate and test accessory relay 2 and any connected external devices to see if the relay is switching incorrectly or being overloaded. The engineer will test the relay output and the PCB drive circuitry.
5. Replace faulty components as required: this may be the flow NTC sensor, the NTC cable/harness, the NTC plug, the accessory relay, or in some cases the main PCB. The engineer will re-test after each repair.
6. If the fault involves cylinder/DHW sensors, the engineer will test those sensors and the actoSTOR electronics plug connections if applicable.
When to call a professional and what to tell them:
- Call a Gas Safe registered engineer whenever D.28 persists after a single safe reset or if you are unsure. This fault involves sensor wiring and electronic components that require specialist testing equipment and safety checks.
- When you call, give the engineer the boiler make/model, serial number, exact fault code(s) shown (D.28 and any accompanying F codes), and any recent changes to the system (new controllers, wiring work, power outages, water leaks).
Urgent signs to report immediately:
- Persistent loss of heating or hot water in freezing conditions
- Any signs of gas smell or CO alarm activation
- Visible water ingress or severely corroded wiring inside the boiler
Summary: You can safely perform non-intrusive checks (reset, pressure, power, condensate and external accessories) but do not open the boiler or attempt wiring or component replacements. D.28 most commonly means a problem with the heating flow NTC or its wiring, or less commonly a relay/PCB issue — a Gas Safe engineer should diagnose and repair the fault.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Vaillant EcoTEC Plus Regular Gas Boiler.