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Mitsubishi Ecodan fault L3

Circulation water temperature overheat protection

Heat pump

What it means

Error code displayed when THW1 detects a temperature ≥ 80ºC for 10 consecutive seconds or THW2 detects a temperature ≥ 80ºC for 10 consecutive seconds. Applicable in DHW/Heating/Cooling/LP/FS/OS modes.

Likely causes

  1. Insufficient system head
  2. Reduced flow in primary water circuit due to faulty pump, insufficient air purge, blocked strainer, or leak in water circuit
  3. Valve operation fault
  4. 2-way valve (local supply) actuator fault
  5. 3-way valve actuator fault
  6. Booster heater relay (BHC1, BHC2, BHCP) operating fault
  7. Power supply voltage increase
  8. THW1 or THW5 has become detached from its holder
  9. THW1 or THW2 fault
  10. FTC board failure

Where to start

  1. Refer to table in section 10-6 to determine if system pump meets requirements
  2. Check circulation pump (see 10-6 for how to check)
  3. Open purge valve to remove trapped air

The full answer, free

  • 15 more checks in order
  • The readings and component tests from the manual
  • How to put it right, step by step
  • The manual page for this fault (page 47), right under the answer
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Safety: Isolate the outdoor unit and any backup heater, and give the inverter capacitors time to discharge before you open the electrics. Anything on the refrigerant circuit needs F-gas certification.

From the Mitsubishi manual for the Ecodan. Check the model on the data plate before you order parts.

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