Overview
The S.34 status code on Vaillant EcoTEC Pro / EcoTEC Plus indicates a problem related to the boiler's heating-mode frost protection. Frost protection is a built-in function that will automatically run the boiler (and the circulation pump) if the heating flow temperature falls below a low threshold (typically around 5 °C) so pipework does not freeze. S.34 means the boiler has detected that the frost-protection activity cannot operate correctly or has found an abnormal condition while trying to protect the heating circuit. There are several common causes: the boiler tried to run frost protection but the pump did not circulate water, the flow/return temperature sensors are not reporting correctly, the heating circuit pressure is too low or there is no water flow through the system (closed service valves, airlock, blocked pipe/valve), or an electronics/communication fault prevented the frost protection sequence. Severity is moderate to high because a persistent failure leaves the heating installation at real risk of freezing and consequential damage in cold weather. Some basic checks and resets are safe for a competent homeowner, but diagnosing and repairing pump faults, sensors, wiring, or internal electronics should be left to a qualified Gas Safe / registered heating engineer.
Possible Cause: Heating mode frost protection error
Troubleshooting Steps
Safety precautions:
1. Do not open the boiler casing, attempt gas or mains electrical repairs, or work on sealed components unless you are a qualified engineer. Incorrect repairs are dangerous. If in doubt, turn the boiler off at the mains and call a qualified engineer.
2. Keep the gas isolator cock fully open when attempting basic checks that require the boiler to operate. If you shut the gas off for safety, do not attempt to relight or service internal components yourself.
3. If freezing is occurring or likely, take immediate measures to protect vulnerable pipework (insulate or keep taps dripping) while arranging a professional repair.
Initial homeowner checks you can safely do:
1. Note the exact fault code (S.34) and any other messages on the display; take a photo for the engineer.
2. Check the boiler is powered and the main switch is on. If the display is blank, check the fuse/consumer unit and the boiler isolation switch.
3. Check the boiler pressure gauge. Normal cold pressure is usually around 1.0–1.5 bar. If pressure is very low, top up the system via the filling loop only if you are comfortable doing so and know how; otherwise contact your installer. Frost protection cannot work if there is insufficient water in the system.
4. Check the heating is not set to 'Heating off' or Summer mode on the boiler and on any external controller/thermostat. Ensure the programmer/room thermostat is not inhibiting the boiler.
5. Listen for the circulation pump when the boiler attempts frost protection (you may need to reset to see this happen). Feel the pipes near the boiler after a few minutes to see if they warm. If the pump is silent and should be running, do not force it; this indicates a pump or electrical fault.
6. Ensure heating flow and return service valves (if fitted) are in the open position so water can circulate.
7. Press the fault-clear/reset button: for ecoTEC Plus the manual indicates pressing the fault clearance key for one second will attempt another ignition/operation. If the code clears and frost protection runs, monitor the boiler — if the fault returns repeatedly, stop and call an engineer.
Specific diagnostic steps and fixes (homeowner level):
1. Reset and observe: Press the fault clear/reset button once (hold for the duration stated in your manual) and watch if the boiler enters frost protection and warms the flow pipe. If it works and stays working, monitor for recurrence.
2. Check system pressure and top up if safe to do so: If pressure is low, use the filling loop to bring pressure into the normal range, then reset the boiler and observe. Low water will prevent circulation and cause protection faults.
3. Purge obvious air: If radiators or the heating system were recently serviced, airlocks can prevent circulation. Bleed radiators if you know how and re-check pressure. If unsure, ask an engineer.
4. Verify external controls: Ensure any room thermostats, timers, or smart controls are not stopping the boiler from running for frost protection. Some controls override the boiler — consult the controller instructions.
When to call a professional (and why):
1. Call a Gas Safe / registered heating engineer if the fault remains after the above checks, especially if the pump does not run, the display shows additional error codes (F.xx), sensors appear faulty, or the boiler repeatedly locks out after reset.
2. Do not attempt to open the boiler cover or touch internal wiring, sensors, gas valves, or PCB components. Troubleshooting sensor faults, pump replacement, valve repairs, or electronic faults requires specialist tools and certification.
3. If pipes show signs of freezing or you suspect frozen pipework, call an engineer urgently to avoid burst pipes.
What to tell the engineer:
1. Provide the exact fault code (S.34), any other displayed codes, and the times the fault occurred. Share the photo you took of the display if possible.
2. Tell them what checks you carried out (pressure top-up, resets, radiator bleeding, thermostat checks) and the boiler model (EcoTEC Pro / EcoTEC Plus).
Summary: S.34 means the boiler's frost-protection sequence has detected a problem preventing it from protecting the heating circuit. Perform safe initial checks (power, gas on, pressure, service valves, reset, controller settings) but contact a Gas Safe registered heating engineer for any pump, sensor, or internal/electrical faults or if the fault repeats.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Vaillant EcoTEC Pro / EcoTEC Plus.