E05
Thermona
Flow Temperature Sensor (NTC) Fault
Boiler displays E05 and does not fire, or fires briefly then shuts down.
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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
Open circuit or shorted flow NTC sensor, Loose NTC wiring harness on Thermona PCB, Corroded NTC connector, Faulty main PCB input
How to Fix / Troubleshooting
Safety: Isolate power before removing the boiler front cover.
- Power cycle: Turn the boiler off for 30 seconds and back on. Sometimes transient sensor errors clear after a reboot.
- Check wiring (visual): Remove the front cover and locate the flow NTC sensor on the primary flow pipe. Inspect the cable and connector for damage or disconnection.
- Connector reseat: Gently unplug and replug the NTC connector on both the sensor and PCB (if accessible) to clear oxidation.
- Resistance test (engineer level): With power off, disconnect the NTC and measure its resistance with a multimeter. Compare to Thermona temperature/resistance chart. Replace if out of tolerance or open/shorted.
- PCB check: If a known-good NTC still reports E05, the PCB sensor input may be defective and the main control board may need replacement.
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Repair Difficulty
Medium
3/5
Required Part
Flow NTC Sensor
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