E4
Eurotronic
Output Relay / Valve Drive Fault
Thermostat shows E4 and cannot control the heating valve or boiler relay.
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Safety Warning
This repair may involve working with high voltage components or water connections. Always unplug the appliance before removing any panels.
If you are not confident in your ability to perform this repair safely, we strongly recommend contacting a professional technician.
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Possible Causes
Stuck relay contacts on PCB, Burned relay coil, Short circuit on valve output terminals, Overcurrent protection triggered on output stage
How to Fix / Troubleshooting
Safety: Isolate power at the breaker and verify with a tester before touching any 230V wiring.
- Disconnect load: Remove the wires going to the boiler or actuator from the thermostat output terminals. Power the thermostat and see if E4 clears. If it does, the external load may be shorted or drawing too much current.
- Check actuator/boiler input: Measure resistance of the connected valve actuator or boiler control input. Compare with manufacturer specs. Replace any actuator that is shorted or has abnormally low resistance.
- Inspect terminals: Look for signs of overheating, melted plastic, or carbonization around the relay output terminals. Clean light oxidation; replace the thermostat if terminals are burned.
- Test relay (advanced): With power off, gently tap the thermostat housing. If the relay intermittently works, contacts may be sticking—replacement of the thermostat is recommended.
- Replace thermostat: If E4 remains with no load connected, the internal relay or triac on the PCB is defective. Replace the complete Eurotronic thermostat.
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Repair Difficulty
Hard
4/5
Required Part
Output Relay / Valve Driver Stage
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