Overview
D.19 on a Vaillant ecoTEC Plus regular boiler is a diagnostic indication related to the heating circulation pump — specifically to the operating modes of a two‑speed heating pump. It commonly appears when the boiler detects that the pump is not operating as expected (for example wrong speed selection, insufficient flow, or an electrical/control inconsistency between the pump and the boiler electronics). Causes range from an airlock or blockage in the heating circuit, closed service valves, or a stuck/failed pump, to wiring/connector faults or a control/PCB problem that isn’t commanding the correct pump speed. Severity is moderate. The boiler will often lock out or reduce operation to protect itself, which typically results in loss of central heating (and may affect hot water on systems that rely on boiler circulation). It is not usually an immediate safety risk like a gas leak, but continuing to force the boiler to run with a circulation problem can cause overheating of the primary exchanger or damage to the pump. Some basic checks are suitable for a competent homeowner (pressure, service valves, bleeding radiators, simple reset). However, diagnosing pump electrical faults, replacing the pump, replacing wiring, or changing PCB settings requires a qualified Gas Safe engineer and should not be attempted by an unqualified person.
Possible Cause: Operating modes of the two speed heating pump
Troubleshooting Steps
Safety precautions
- If you are unsure at any stage, switch the boiler off at the external isolator and at the boiler main switch and call a Gas Safe registered engineer. Do not open the casing of the boiler or touch gas or live electrical parts. Always isolate electrical supply before any physical inspection inside the boiler and do not attempt to repair gas or sealed components yourself.
Initial homeowner checks you can safely do
1. Note the exact code and behaviour: is D.19 persistent, intermittent, or appears only when heating is called for? Also note any recent work or power cuts.
2. Try a reset: press and hold the boiler power/reset button for about 5 seconds to clear the fault. If it reappears immediately or shortly after, further checks are needed.
3. Check system pressure: view the pressure gauge on the boiler. Typical cold system pressure is about 1.0–1.5 bar. If pressure is very low (below ~0.8 bar) top up the system according to your boiler instructions and then reset. Low pressure can cause circulation problems.
4. Confirm service valves: ensure the heating circuit flow and return service valves (if fitted) under the boiler are fully open (in line with the pipe). If they are closed or partly closed the pump may not circulate.
5. Bleed radiators and check for airlocks: air in the system can prevent circulation. Bleed radiators starting from the highest in the house to remove trapped air, then check pressure again and top up if needed.
6. Listen and observe: when heating is called for, listen at the boiler for pump noise/operation. A properly operating pump will run and you may hear a change in sound when the demand changes. If the pump is silent or makes grinding/strange noises this suggests a mechanical/electrical fault.
7. Check external controls: make sure the room thermostat/timers/controllers are demanding heat and the boiler is not in summer/off mode or in a locked/installer menu restricting pump speeds.
If the basic checks do not clear the fault (or the pump is noisy, completely silent, or D.19 returns): professional diagnostic steps (Gas Safe engineer required)
1. Isolation and inspection: engineer isolates electrical and gas supplies, removes the boiler front cover and inspects the pump, wiring harnesses, connectors and PCB for loose connections, corrosion or moisture.
2. Electrical tests: measure supply to the pump, check switching between low and high speeds, verify the pump motor current and resistance, and test continuity of wiring and connectors. They will also check any coding resistor or soldered links used by the PCB to select pump speed.
3. Flow checks: verify that the pump is actually producing flow and that there is no internal blockage or airlock in the appliance or system. If required the engineer will purge the circuit, check service valves again, and may carry out a power‑flush if system debris/blockage is suspected.
4. Component replacement: if the pump motor is seized, worn, or electrically faulty the pump unit will need replacing. If the PCB or pump speed control circuitry is faulty the PCB or its components may need replacement. If thermistors/NTCs or sensors related to flow/return temperatures are implicated these will be tested and replaced if defective.
5. Final checks: after repair or replacement the engineer will reassemble, refill and purge the system, set the correct pump speed/controls in the installer menu (if required), and run the boiler through several demand cycles to confirm the fault is cleared.
Important notes and when to call a professional
- Do not attempt to dismantle the pump, open the boiler casing, or work on gas/electrical connections yourself. These actions require a Gas Safe registered engineer. If D.19 persists after the simple homeowner checks (reset, pressure top-up, bleeding radiators, open service valves) call a Gas Safe engineer and report the exact fault code and what you have already checked. This helps the engineer arrive prepared and reduces diagnosis time.
- If you observe leaks, burning smells, or heavy smoke, isolate the boiler and call emergency services/Gas Safe immediately. Otherwise, treat D.19 as an urgent but non‑emergency fault needing professional attention to avoid further damage to the boiler and heating system.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Vaillant EcoTEC Plus Regular Gas Boiler.