Overview
D.1 on a Vaillant EcoTEC combi typically means the boiler is in a pump-overrun condition for heat mode — the pump is being kept running after the burner has stopped to cool the primary heat exchanger. On modern boilers this is a normal protection feature designed to remove residual heat and prevent the heat exchanger from boiling or overheating. A normal overrun usually lasts only a few minutes (commonly 5–10 minutes) and then clears by itself. When D.1 is persistent or the pump keeps running for a long time, it indicates a fault or incorrect signal rather than normal operation. Common causes include a stuck thermostat/timer or zone valve calling for heat, a faulty pump overrun thermostat or flow-temperature sensor (NTC), incorrect wiring (often after a pump change), or a failing PCB. Severity ranges from not urgent (normal short overrun) to important (continuous pump operation wastes energy, causes noise, and can accelerate wear). If the code clears in the normal time and there are no other symptoms you can usually monitor it yourself; if the overrun is prolonged, intermittent or accompanied by other fault codes, a Gas Safe registered engineer is needed.
Possible Cause: Water pump overrun – time for heat mode
Troubleshooting Steps
Safety first: do not open the boiler casing, work on gas or mains electricity, or attempt internal PCB or gas-valve repairs yourself. If you are in any doubt, switch the boiler off at the isolator and the gas supply and call a Gas Safe engineer. Basic homeowner checks and observations are safe to do and often help the engineer diagnose the problem faster.
1) Observe and note behaviour
- Record exactly what you see: when the D.1 appears, how long the pump runs, whether the code clears after 5–10 minutes, and which display symbols appear (radiator for CH, tap for DHW, flame for burner). Note if the pump runs when you haven’t used hot water or heating.
2) Initial, safe checks you can do
- Give the boiler time: turn heating off at the programmer/stat or set to a long off period, then watch for 10–15 minutes to confirm whether the pump overrun clears normally.
- Check your room thermostat, programmer/timer and any smart controls to ensure they are not calling for heat. Also check radiator TRVs are shut if you expect no demand.
- If you recently had work done (new pump fitted or wiring changed), tell the engineer — incorrect pump wiring is a common cause of continuous run.
- Look at boiler pressure on the gauge. Very low pressure (<0.8 bar) can create flow/pressure faults; if you know how to use the filling loop and are comfortable doing so, you can top up to the correct pressure (check boiler manual). If unsure, leave this for the engineer.
3) Specific diagnostic checks you can safely perform
- Restart test: switch the boiler off at the front panel or isolator for 10 minutes, then switch it back on and recreate a hot water or heating demand. Watch the sequence: does the burner run, then stop with pump still running and D.1 displayed? Does the burner restart incorrectly? Note any temperature readings rising rapidly.
- Confirm whether the overrun is CH (heating) or DHW related by watching the symbols when the pump runs. DHW overrun with frequent short pump activations and no user demand can indicate a faulty flow sensor or wiring.
- Visually check external wiring and terminals you can see without removing the casing. Look for obvious loose connections, damaged wires or water ingress around the pump area. Do not touch live terminals.
4) Actions and what they indicate
- If the overrun clears in 5–10 minutes and there are no other faults: this is normal; monitor for recurrence.
- If the pump runs continuously, or the D.1 stays for a long time, or you have repeated starts/stops or other fault codes: this indicates an electrical/control or sensor problem (timer/thermostat, motorised valve stuck, faulty NTC/flow sensor, PCB fault or incorrect pump wiring).
- If a new pump was fitted and the problem began afterwards: isolate the boiler and call the installer/engineer — incorrect pump wiring is a likely cause and requires an engineer to correct safely.
5) When to call a Gas Safe engineer (and what they will check)
- Call a Gas Safe engineer if the overrun doesn’t clear in the normal time, the pump runs continuously, or you see other error codes or signs of overheating. The engineer will test flow and return NTC sensors, check pump operation and wiring, inspect motorised/three-port valves, test the pump-overrun thermostat/timer circuitry and the PCB, check pressure-switch and safety limits, and carry out safe repairs or part replacements as required.
Final note: short pump overrun is normal and protective. Persistent or long-running pump overrun, repeated automatic attempts to run the pump, or other fault codes should be investigated by a qualified Gas Safe engineer rather than tackled as DIY. If you ever suspect overheating, gas or electrical faults, switch the boiler off and get professional help immediately.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Vaillant EcoTEC Gas Boiler.