Overview
S.03 on a Vaillant EcoTEC Pro / EcoTEC Plus is a status code that indicates an ignition sequence error in heating mode. That means the boiler tried to start the burner to satisfy a heating demand but the ignition process did not complete or the flame was not detected. It is essentially a failure to light during a heating call rather than a temperature or sensor short-circuit code. This error can be caused by a number of issues: lack of gas supply or low gas pressure, blocked air intake or flue, failed ignition electrode or ignition lead, defective ignition transformer, faulty gas valve or gas valve control, interrupted flame detection (ionisation) or earthing issue, fan or condensate problems, wiring or PCB faults, or a blocked condensate drain (especially in freezing weather). Severity ranges from inconvenient (no heating/hot water) to potentially dangerous if there is a gas smell or obvious gas supply fault. Some basic, non-intrusive checks can be done by a homeowner, but internal boiler repairs and any work involving gas must be carried out by a Gas Safe registered engineer.
Possible Cause: Heating mode – ignition sequence error
Troubleshooting Steps
Safety precautions:
1) If you smell gas: do not switch electrical items on or off, open windows, leave the property immediately and call your emergency gas number. Do not attempt any checks besides evacuating and calling the emergency service.
2) If in doubt at any stage, stop and call a Gas Safe registered engineer. Do not open the boiler casing, touch gas components, wiring, or internal electronics.
Initial checks a homeowner can safely do:
1) Note the exact error code display and whether it is S.03 only or accompanied by other codes. Take a photo or write it down.
2) Check whether other gas appliances are working (hob, fire). If all gas appliances are out, check your gas meter or supplier (possible supply interruption). Contact your gas supplier if you suspect a supply cut.
3) Check the boiler’s electrical supply and switches: confirm the fused spur or isolation switch is on and the boiler has power. If the boiler display is dead, investigate power/fuses before further checks.
4) Try a single controlled reset of the boiler following the user manual (use the boiler reset button or switch the fused spur off for about 30–60 seconds and back on). Do not repeatedly reset a locked-out boiler; one reset to confirm persistence is enough.
5) Check that room thermostats/timers and any weather compensation controls are requesting heat (ensure it is not simply no demand). Verify the programmer is set to heating mode and the room thermostat set above room temperature.
6) Check boiler water pressure on the gauge; most systems need roughly 1–2 bar. Very low pressure can cause faults—refer to manual for topping-up instructions if you are familiar with that safe procedure.
7) Inspect external flue terminal and air intake for obvious blockages (bird nest, leaves, debris) and check condensate pipe for freezing or blockages (thaw frozen condensate with warm water/insulation if safe to do so).
Diagnostic and safe next steps (homeowner scope):
1) Observe and listen during a heating demand: does the fan run, do you hear ignition clicks, does it attempt to light and then lock out? This information helps the engineer diagnose ignition vs flame-loss.
2) Check whether domestic hot water (DHW) demand works independently. If DHW works but heating fails, note that—it narrows the fault to heating circuit or controls.
3) If you have a condensate pump fitted, check it is not blocked and the float is not stuck.
4) Gather history to give to the engineer: when the fault started, any recent power cuts, frozen conditions, maintenance done, whether the fault is intermittent or permanent, and any other error codes seen.
What requires a professional:
1) If the S.03 error persists after the safe checks above, contact a Gas Safe registered engineer. The typical engineer diagnostics will include verifying gas pressure and supply, inspecting ignition electrode and lead, testing the ignition transformer, checking flame sensing/ionisation circuit and earthing, testing gas valve operation and gas valve control wiring, inspecting the fan and flue for correct operation and blockages, checking the PCB for fault codes/parameter settings, and removing or thawing blocked condensate drains safely.
2) Repairs such as replacing electrodes, ignition transformer, gas valve, PCB or any internal wiring, or conducting gas pressure adjustments and flue modifications must be done only by a qualified engineer.
What to tell the engineer when you call:
1) Give the exact model (Vaillant EcoTEC Pro / EcoTEC Plus), the displayed code S.03, whether the problem is heating-only or affects hot water, any recent resets and their result, whether other gas appliances work, and whether there was freezing weather or a power/gas interruption prior to the event.
Final note:
A transient S.03 can sometimes be cleared by a single reset after a temporary gas supply or airflow issue, but repeated ignition failures indicate a component or supply fault that needs professional attention. For safety and to ensure correct repair and testing, always use a Gas Safe registered engineer for any internal inspection or repair.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Vaillant EcoTEC Pro / EcoTEC Plus.