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Vaillant Gas boiler fault F.75

Pump fault/water deficiency

Gas boiler

What it means

A pump malfunction or water deficiency has been detected. Further diagnosis available via diagnostics code D.149.

Likely causes

  1. Pump blocked
  2. Electrical pump fault
  3. Pump dry run
  4. Pressure sensor fault
  5. No feedback from the pump
  6. Incorrect pump detected
  7. Flow rate at the end of the purge programme is insufficient
  8. Closed stopcocks in the hydraulic installation
  9. Closed thermostatic valves on radiators
  10. Insufficient filling pressure in the heating installation
  11. Air remaining in the system preventing adequate flow
  12. Malfunction
  13. D.149 = 1, pump blocked alarm
  14. Defective pump

Where to start

  1. Read parameter D.149 to determine the specific circulation fault value
  2. Match D.149 value to the corresponding cause from the list: 0=No fault, 1=Pump blocked, 2=Electrical pump fault, 3=Pump dry run, 5=Pressure sensor fault, 6=No feedback from the pump, 7=Incorrect pump detected, 8=Flow rate at end of purge programme insufficient
  3. Ensure that all of the stopcocks in the hydraulic installation are open

The full answer, free

  • 9 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 62), right under the answer
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Safety: Isolate gas and power before you open the case. If you have been into the gas side, do a tightness test, and a combustion and flue check before you hand back. Gas Safe (UK) or RGI (Ireland) work only.

From the Vaillant manual for the Gas boiler. Check the model on the data plate before you order parts.

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