Heat pumps · Error codes · Vaillant
M.33Vaillant · flexoTHERM exclusive (VWF ../4)
M.33 on a Vaillant flexoTHERM exclusive: Fan unit: Cleaning required
Short answer
On a Vaillant heat pump, M.33 means: Fan unit: Cleaning required. The heat pump keeps running. An engineer is needed to fix the cause.
Reviewed 23 September 2026 · 1 source
What it means
The manual says: Fan unit: Cleaning required.
Part involved: Fan unit: Cleaning required.
Likely causes:
- Air inlet or air outlet of the air-tobrine heat exchanger contaminated; Heat transfer from the air to the brine heat transfer medium is too low; Brine pipes inverted; Air in the environment circuit; Defective de-icer; Fan noise reduction mode activated; The fan unit is de-iced more frequently than is required. The efficiency of the heat pump is reduced. Loosen and clean any dirt from the fan unit (air-to-brine heat exchanger).
Is it safe?
| Does the heat pump stop? | No |
|---|---|
| Severity | Warning |
| Engineer needed? | Yes |
The heat pump keeps running.
Resetting itself: clears when condition resolved.
Manual reset: engineer resets maintenance message.
What you can check
- Note the full code and text shown on the display (and the date/time it appeared). Check the outdoor unit's air inlet and outlet are not blocked by leaves, debris, snow or ice build-up and clear any loose debris from around it (do not open the casing, poke the fins or use tools). 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
- Check that the connections are correctly assigned to the brine lines; Purge the environment circuit; Check de-icer (sensor/actuator test?); Reduce noise reduction mode; Completely deactivate fan noise reduction mode.
Who to ask for: Vaillant-trained heat pump installer/engineer (MCS-certified installer).
Certification: MCS installer certification; Vaillant product training recommended.
What you might notice
Sources
- flexoTHERM exclusive VWF 57/4 - 197/4 400V installation and maintenance instructions ↗ · 0020213394_05 · page flexoTHERM 400 V 0020213394_05 p.50; flexoTHERM 230 V 0020213395_04 p.45 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026