Overview
D.12 on a Vaillant EcoTEC is a diagnostic code linked to the cylinder charging pump controlled via an accessory module (an external pump or controller fitted to charge a hot water cylinder). As a diagnostic message it can simply indicate that the boiler is attempting to operate the cylinder charging circuit through an accessory module — but it can also appear when there is a problem with that module, its wiring, or related cylinder sensors (NTC) and connections. If the code appears together with functional faults (no hot water from the cylinder, or additional F‑codes such as flow/cylinder NTC errors), it usually points to an electrical/communication or sensor wiring issue rather than a direct gas safety fault. Severity ranges from low to medium: if the code is only reporting normal operation of an accessory pump there is no immediate danger, but if it indicates a failure of the pump, module or sensor it will affect hot water delivery and should not be ignored. Because the accessory module, pump wiring and NTC sensors connect to the boiler control electronics and relate to the hot water safety controls, diagnosis and repairs beyond basic checks should be carried out by a Gas Safe registered engineer or qualified heating technician.
Possible Cause: Cylinder charging pump via accessory module
Troubleshooting Steps
Safety precautions:
- Do not attempt to access or repair gas or mains electrical parts yourself. Turn off the boiler using the on/off switch or the electrical isolator if you need to power it down for basic checks. If you suspect a gas leak, evacuate the property and contact the gas emergency number immediately.
- If you are not comfortable with electrical checks or working around pumps, stop and call a Gas Safe engineer.
Initial homeowner checks you can safely do:
1) Note any other error codes on the display (F codes) and any symptoms (no hot water to the cylinder, strange noises, leaks). Make a note of them before you reset.
2) Reset the boiler once using the manufacturer reset procedure. If the code returns immediately or shortly after, proceed with the following checks.
3) Check external power and obvious signs: ensure the boiler has power, the external accessory module (if visible) is mounted and powered, and the external pump has power (check switches or fused spur if present). Do not open or remove covers that expose mains wiring.
4) Listen and feel: when the boiler is calling to charge the cylinder, listen for the external pump running or feel for vibration in the pump body or pipework. If the pump does not run when it should, that indicates either the pump, its power supply, or the control signal is faulty.
5) Visual inspection: with the boiler and any accessory isolated electrically, look for loose connector plugs at terminals you can safely access (e.g., accessible plug-in connectors on the external module). Look for water ingress, corrosion or damaged cables. Do not disconnect wiring inside the boiler or on gas components.
Specific diagnostic steps (for an engineer if the basic checks fail):
- Check for associated sensor faults: cylinder and flow NTC sensors and their plug/connections can cause faults that show with D codes plus F codes. An engineer will measure sensor resistance (typical NTC values are in the kilo‑ohm range — for example some NTCs are around 12 kΩ at 20°C) and check for short circuits or open circuits in the wiring harness.
- Verify accessory module communications and power: the accessory module must receive the correct control signal and auxiliary power from the boiler. An engineer will test the control outputs from the boiler PCB and the supply to the external pump or module.
- Inspect and test the pump: the pump may be electrically dead, seized, or obstructed. An engineer will isolate and test the pump, check strainer/filters on the pump circuit, check isolation valves are open, and ensure the pump rotates freely.
- Inspect wiring harnesses and plugs for short circuits, damaged insulation, or poor connections between the cylinder NTC, accessory module, and main PCB. Repair or replace damaged loom/cables as required.
- If sensor or wiring faults are confirmed, the NTC sensor, wiring harness, accessory module or main PCB may need replacing. These are specialist tasks for a registered engineer.
When to call a professional and what to expect:
- Call a Gas Safe registered heating engineer if the code persists after reset, if the pump does not run when expected, if you find damaged wiring, if additional F codes appear, or if you lack the tools/skills to safely investigate electrical or sensor faults.
- The engineer will perform measured checks on sensors and wiring, test the accessory module and pump, and repair/replace any failed components. They will also check for related faults (e.g., NTC short/open, eBUS or module communication errors).
Important final notes:
- Do not attempt internal boiler repairs, PCB or gas valve work yourself. Incorrect work can cause gas leaks, carbon monoxide risk, or electrical danger.
- If the boiler is producing no hot water or you have safety concerns, isolate the appliance electrically and contact a Gas Safe engineer promptly. Provide the engineer with the exact error code(s) and any symptoms you observed to speed diagnosis.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Vaillant EcoTEC Gas Boiler.