Overview
D.30 on a Vaillant EcoTEC is a diagnostic message indicating a control signal situation involving the gas valve assembly — effectively the boiler electronics have detected both gas valve control signals or an abnormal condition with gas-valve control. In practice this fault often appears when the safety cut-out (SCO) has interrupted gas valve control because the boiler has detected a temperature or flow/pressure condition outside safe limits (for example an over-temperature of the flow/return sensors, extremely low water pressure, or an interrupted flow). It can also be triggered by faults in sensors, pump failure, wiring faults, or an internal electronics (PCB) or gas valve assembly fault. Severity: this is a safety-related fault. It can leave the boiler locked out and your heating/hot water unavailable. Because it involves gas control signals and safety cut-outs, it should be treated seriously. Some basic checks can be done by a competent homeowner, but tests and repairs that involve the gas valve, ignition circuitry, sensors or the PCB must be carried out by a Gas Safe registered engineer. Repeatedly resetting the boiler without diagnosing the underlying problem is not recommended.
Possible Cause: Control signal for both gas valves
Troubleshooting Steps
Safety precautions:
1) If you smell gas: do not operate electrical switches, open windows, leave the property immediately and call the gas emergency number for your area. Do not attempt any repairs.
2) If the boiler or pipes are very hot, or you see steam or signs of overheating, turn the boiler off at the programmer and the electrical isolation switch and contact a Gas Safe engineer.
3) Do not open the boiler casing or attempt to work on gas, ignition, or PCB components yourself. Only visual checks of external components and basic user operations are appropriate for a homeowner.
Initial checks a homeowner can do (safe, low-skill):
1) Note the exact error code and behavior (D.30 only, or D.30 plus F.xx codes). How often does it recur? Is heating or hot water affected?
2) Try a single reset: switch the boiler off via the programmer, switch off the electrical isolator for 5–10 minutes, then switch back on and observe. If the fault returns immediately or on first demand, stop attempting resets.
3) Check system water pressure on the boiler gauge. If pressure is significantly below 1.0 bar, follow the boiler manual to top up using the filling loop (only if you are comfortable doing so). Low pressure can cause protection faults.
4) Look for obvious signs: visible leaks around the boiler or pipework, frozen or blocked condensate discharge (in cold weather), or a tripped external safety device.
5) Check central heating controls: make sure the room thermostat, programmer and any smart controls are calling for heat correctly. Also check that radiators are not fully closed and bleed radiators if there is air trapped.
More detailed diagnostic checks you can try (observe safety warnings):
1) Listen for the pump when the boiler is calling for heat. If the pump appears not to run (no vibration or sound) and you are comfortable doing a surface check, the pump may be blocked or electrically disconnected. Do not open the boiler to test electrical supplies.
2) If you have two separate displays or an installer app showing sensor values, note flow and return temperatures or any abnormal readings. A very large temperature difference (hot flow, cool return) can indicate poor circulation or blocked 3-way/diverter valve.
3) Check the condensate pipe outside (if fitted) for blockage or freezing — clear gently if you can do so safely.
4) Check other gas appliances to confirm the mains gas supply is present. If there is no gas supply to the property, contact your gas supplier.
When to call a professional and what they will check (required for most D.30 causes):
1) If the D.30 fault persists after the basic checks above, isolate the boiler (turn off) and call a Gas Safe registered engineer. Do not attempt to replace or adjust the gas valve assembly, PCB, ignition components or internal sensors yourself.
2) Give the engineer: the exact fault code(s), how often it happens, recent servicing history, current system pressure, and any other symptoms (loss of heating, hot water, noises, visible leaks).
3) The engineer will carry out safe gas tightness and control checks, measure sensor resistances (flow/return/water pressure sensors), test the pump and diverter valve operation, inspect wiring and connectors, check for internal overheating or blocked flow, and test the gas valve assembly and PCB. If a gas valve or PCB is suspected they will isolate gas and follow manufacturer procedures for testing and replacement.
Important final notes:
1) D.30 can be a symptom of several different faults: sensor failures, pump or circulation faults, low water pressure, blocked diverter or condensate, wiring issues, gas valve leakiness or PCB faults. Many of the definitive diagnostic and repair steps involve gas and live electrical work and must be done by a Gas Safe engineer.
2) If the boiler shows any signs of overheating, repeated lockouts, or you detect a gas smell, switch the boiler off and seek emergency assistance immediately.
3) Provide the engineer with the fault code, circumstances and any checks you have already performed to speed diagnosis and repair.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Vaillant EcoTEC Gas Boiler.