M.201 on a Vaillant aroTHERM plus: Sensor fault: Cylinder temperature
Short answer
On a Vaillant heat pump, M.201 means: Sensor fault: Cylinder temperature. 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: Sensor fault: Cylinder temperature.
Part involved: Cylinder temperature.
Likely causes:
- Cylinder temperature sensor defective.
Is it safe?
| Does the heat pump stop? | No |
|---|---|
| Severity | Warning |
| Engineer needed? | Yes |
The heat pump keeps running.
Resetting itself: yes - message clears when the condition is resolved (maintenance message reset by engineer via Installer level > Resets).
Manual reset: engineer resets maintenance message after work.
What you can check
- Note the full code and text shown on the display (and the date/time it appeared). Contact your installer or Vaillant Service; do not attempt any repair.
What the engineer will check
- Check the plug contact on the PCB and on the wiring harness; check that the sensor is working correctly and, if required, replace the sensor.
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
What owners and engineers report
7 owner and engineer reports mention this.
- Cylinder or buffer thermostat/sensor problem on the aroTHERM plus (VWL ../6 A): 4 reports
- Hot water not hot enough on the aroTHERM plus (VWL ../6 A): 3 reports
Sources
- Heat pump control interface module (VWZ AI) operating and installation instructions ↗ · 0020291573_01 · page VWZ AI appliance interface 0020291573_01 p.33; uniTOWER VIH QW 190/6 E 0020291525_05 p.59; VWZ MEH 97/6 hydraulic station 0020291544_03 p.51; aroTHERM split hydraulic station VWL ../5 IS 0020257319_05 p.56 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026