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Heat pumps · Error codes · Vaillant
F.753Vaillant

F.753 on a Vaillant heat pump: Connection fault: Freq. conv. n.recogn

Short answer

On a Vaillant heat pump, F.753 means: Connection fault: Freq. conv. n.recogn. The heat pump stops until the fault clears or is reset. An engineer is needed to fix the cause. The wording differs by model range — each is shown below.

Reviewed 23 September 2026 · 2 sources

What it means

On the aroTHERM plus (VWL ../6 A)

The manual says: Connection fault: Freq. conv. n.recogn.

Part involved: Freq. conv. n.recogn.

When it shows: No communication between frequency converter and outdoor control PCB; follows S.581 occurring three times.

Likely causes:

  • Missing communication between the frequency converter and the outdoor unit's control PCB.

On the aroTHERM

The manual says: Connection fault: Frequency converter not recognised.

Part involved: Frequency converter not recognised.

Likely causes:

  • 'Max. duration power supply cut-off' incorrectly set; connection between main PCB and inverter box damaged/broken; inverter box not switched on.

Is it safe?

Does the heat pump stop?Yes
SeverityFault — needs a manual reset once the cause is fixed
Engineer needed?Yes

The heat pump stops until the fault clears or is reset.

Resetting itself: no - the fault remains displayed until the cause is eliminated.

Manual reset: yes - after the cause is eliminated, press the reset button (indoor unit) or reset via the system control; if it recurs after several resets contact customer service.

What you can check

On the aroTHERM plus (VWL ../6 A)

  • Note the full code and text shown on the display (and the date/time it appeared). Check the heat pump supply breaker/isolator (and back-up heater breaker, if separate) at the consumer unit is on; if a breaker has tripped more than once, leave it off and call an engineer - do not open any casing. The operating instructions allow the reset button on the indoor unit (or a reset via the system control) to be pressed once; if the code returns, do not keep resetting - contact your installer or Vaillant.

On the aroTHERM

  • Note the full code and text shown on the display (and the date/time it appeared). Check the heat pump supply breaker/isolator (and back-up heater breaker, if separate) at the consumer unit is on; if a breaker has tripped more than once, leave it off and call an engineer - do not open any casing. Do not keep resetting the unit; report the code to your installer (the operating instructions for this product were not checked for a homeowner reset procedure).

What the engineer will check

On the aroTHERM plus (VWL ../6 A)

  • Check the integrity of the wiring harness and plug connection and, if required, replace them; Check the frequency converter by actuating the compressor safety relay; Read the assigned parameters for the frequency converter and check whether values are displayed.

Who to ask for: Heat pump engineer with electrical competence (qualified electrician for supply/phase faults).

Certification: Competent electrician (BS 7671 / Part P) for supply-side work.

On the aroTHERM

  • Diagnose per cause list in the aroTHERM installer fault table; inform Vaillant technical support (0330 100 3540) for refrigerant-circuit faults.

Who to ask for: Heat pump engineer with electrical competence (qualified electrician for supply/phase faults).

Certification: Competent electrician (BS 7671 / Part P) for supply-side work.

What you might notice

What owners and engineers report

17 owner and engineer reports mention this.

  • No heating on the aroTHERM plus (VWL ../6 A): 8 reports
  • No hot water on the aroTHERM plus (VWL ../6 A): 5 reports
  • Heat pump will not start on the aroTHERM plus (VWL ../6 A): 2 reports
  • No heating on the aroTHERM: 2 reports

Sources

A code explains what the controller detected, not a confirmed root cause.

Reviewed: 23 September 2026