Overview
On a Vaillant EcoTEC Plus Regular, diagnostic code D.12 is associated with the cylinder charging pump as controlled via an accessory module. In practice this means the boiler has detected a communication, wiring or sensor problem related to the pump or the accessory module that controls charging an external hot-water cylinder (this can also be linked to the cylinder NTC sensor or its wiring). The boiler may show D.12 alone or together with other fault codes (for example F.13 / F.91 / flow/return NTC faults) that point to sensor or connection issues. Severity is medium: this fault usually prevents correct charging/circulation of the hot-water cylinder so you may lose domestic hot water or have poor cylinder performance, but it is not normally an immediate gas safety emergency. However, because the underlying causes commonly involve temperature sensors (NTCs), wiring, the accessory module and sometimes the main PCB or pump wiring, diagnosis and repair generally requires a qualified Gas Safe engineer. There are a few safe checks a homeowner can do, but do not attempt to strip the boiler, replace sensors, or work on gas/electrical connections yourself.
Possible Cause: Cylinder charging pump via accessory module
Troubleshooting Steps
Safety precautions:
1) If you smell gas, leave the property immediately, shut off the gas supply if it is safe to do so at the meter, do not operate electrical switches, and call the gas emergency number and a Gas Safe engineer.
2) Before opening the boiler cover, switch the electrical supply to the boiler off at the isolator or consumer unit. Do not attempt internal electrical/gas repairs unless you are a Gas Safe registered engineer.
3) If there is a visible water leak or flooding, isolate the electrical supply and the mains water feed and call a professional.
Initial homeowner checks (safe, non-intrusive):
1) Note and photograph the exact code(s) on the display and whether D.12 appears with other faults (e.g. F.13, F.91, F.10). Record when it started and any symptoms (no hot water, pump noise, leaks).
2) Try a simple reset: switch the boiler off and back on at the isolator, wait a minute, then restart. If the code returns immediately or repeatedly, do not keep resetting; this indicates a persistent fault.
3) Check mains power and any fused spur that feeds an external accessory module or external cylinder pump. Ensure fuses and RCDs/trips in your consumer unit haven’t triggered.
4) Listen and look: on a call for hot water, can you hear the cylinder charging pump run? Is the external pump’s isolator switched on? If the pump never runs but should, that supports an accessory/power/communication fault.
5) Check boiler water pressure on the gauge. Low system pressure can cause other faults; top up only if you know how and the boiler manual instructs you to do so.
6) Look for obvious loose plugs or damaged cables visible externally between any accessory module and the boiler; do not unplug or probe inside the boiler while live.
If the above checks do not clear the fault or if you find loose/damaged wiring, stop and call a qualified engineer. Do not attempt internal repairs.
What a Gas Safe engineer will do (diagnostic and fix steps you can expect):
1) Review boiler and accessory error history and re-create the fault to observe behaviour during a call for cylinder charge.
2) Isolate electricity and open the boiler to inspect the accessory module connections, eBUS/communication connectors, NTC sensor plugs and wiring harness for loose plugs, corrosion, pin damage or water ingress.
3) Measure NTC sensor resistance (NTC values vary by model but the flow/return NTCs are commonly checked at ~12 kΩ at 20°C) and test continuity of the sensor cable harness. Replace NTC or harness if defective.
4) Test the accessory module and external pump supply (voltage and switching) and check the pump itself for electrical continuity and mechanical operation. Replace or repair the external pump or its fused spur if faulty.
5) Check the PCB eBUS/communication circuits and any coding resistor connections; where short-circuits or PCB faults are found the board or module may need replacement.
6) If corrosion, moisture or a short in a cable or connector is present, repair/replace the affected loom, connector or module and then clear the fault and confirm correct operation of cylinder charging.
Important notes and final advice:
- Many of the D.12 root causes involve sensors, wiring harnesses, accessory modules or the boiler PCB. These are not safe to repair without Gas Safe certification and the correct test equipment.
- Keep a record of the error code, any other simultaneous codes, and what checks you have already performed; this speeds up the engineer’s diagnosis.
- If you are without hot water and need an interim solution, a Gas Safe engineer can advise safe temporary measures. Avoid attempting to bypass any safety devices.
Call a Gas Safe registered engineer if the simple checks above do not resolve the fault or if you find damaged wiring, a non‑responsive pump, persistent sensor faults, or any sign of gas or water leakage. They will have the tools to test NTC resistances, continuity, pump operation and PCB behaviour and to carry out any necessary replacements safely.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Vaillant EcoTEC Plus Regular Gas Boiler.