Overview
D.27 on a Vaillant EcoTEC is a diagnostic indication relating to the accessory relay 1 on the boiler control PCB. That relay is used to switch an external accessory (for example a zone pump, motorised valve, external room thermostat/programmers, or a cylinder charger). The code can mean the relay is being commanded to switch but cannot, the relay is stuck, there is a short or interruption in the wiring to the accessory, or there is an electronic fault on the PCB that controls that relay. Severity ranges from a low-to-medium operational fault to a fault that prevents parts of the heating system from working (for example a zone not calling for heat or DHW not being switched). It is primarily an electrical/control fault rather than a gas/flame safety fault, but electrical faults on the PCB can cause lockouts or be symptomatic of moisture or other failures that need professional attention. Basic checks can be done by a homeowner (reset, check external controls and visible wiring), but diagnosing and repairing the relay, wiring harnesses, or replacing PCB components requires a qualified engineer (Gas Safe registered for gas-related systems and a competent electrician for 230V parts). Do not attempt internal repairs on the boiler if you are not qualified.
Possible Cause: Switching accessory relay 1
Troubleshooting Steps
Safety precautions:
1) Before doing anything, switch the boiler off at the normal ON/OFF controls and at the isolator/fused spur. If you are going to remove covers or inspect internal wiring you must isolate mains power to the boiler and wait a few minutes for capacitors to discharge. If you are not competent with electrical equipment do not open the boiler casing.
2) Do not attempt gas-side work; any internal boiler electrical or gas repairs must be carried out by a Gas Safe engineer.
Initial homeowner checks you can do (no internal access required):
1) Note exactly when the D.27 is shown and whether any other fault codes appear at the same time. Take a photo of the display.
2) Try a full reset: switch the boiler off at the programmer and at the isolator, wait 60 seconds, then power back up and call for heating or hot water. Note whether D.27 reappears immediately or only when a particular zone/programme calls.
3) Check external controls: room thermostat, programmer, wireless relay modules, smart hubs, or cylinder stat that control the accessory. Ensure they have power and are calling correctly. If you have battery-powered controls, check batteries.
4) Inspect any visible external wiring and accessory devices (for example external pump or motorised valve) for obvious damage, damp, corrosion, or loose connectors. Do not touch exposed conductors while powered.
If you are comfortable with basic isolation and the boiler isolation is turned off, further diagnostic checks (only if competent):
1) With power isolated, open any external wiring junctions you have access to (not inside the boiler) and ensure plugs and connectors are fully seated and free of corrosion. Re-seat connectors.
2) If the accessory is external and you can safely isolate it, disconnect the accessory from the boiler wiring harness then restore power and test. If the D.27 no longer appears when the accessory is disconnected, the fault is likely in the accessory (short, jammed motor) or its cable. If the code persists with the accessory disconnected, the problem is likely on the boiler side (relay or PCB).
3) Do not measure internal PCB voltages or resistance unless you are trained. If you do have the skills and the boiler is isolated, you can visually inspect the PCB area around the relay for burn marks, bulging components or water ingress. Evidence of moisture or physical damage indicates the boiler needs professional attention.
When to call a professional and what to tell them:
1) Call a Gas Safe registered engineer (and an electrician experienced with boilers if advised) if the fault persists after the simple checks above, if you are unable to safely access/disconnect the accessory, if you find signs of moisture or PCB damage, or if the boiler is locked out and you have no heating/hot water.
2) Tell the engineer the exact fault code (D.27), the boiler model and serial number, what external accessory is fitted (e.g., external pump, zone valve, cylinder stat), what you tried (reset, disconnecting accessory, photos), and whether any other fault codes occur simultaneously. Provide photos if possible.
Likely professional actions (for information only):
1) The engineer will electrically test the accessory circuit, wiring harness and relay output using a multimeter and will check for earth/short faults on the output. They will also check for moisture on the PCB and surrounding components.
2) If the accessory or its cable is defective they will repair/replace that part. If the relay on the PCB is faulty or the PCB is damaged they may replace the PCB or the relay module. They will also check system earthing and any related sensors if required.
Final notes:
1) Avoid operating the boiler in a compromised condition. If the boiler is still providing heating/hot water but is showing D.27, understand an accessory may not operate correctly and monitor the system until an engineer arrives.
2) Do not attempt internal electrical repairs or PCB replacement yourself; these require a qualified engineer for safety and to maintain any warranty or insurance cover.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Vaillant EcoTEC Gas Boiler.