I.201 on a Glow-worm GeniaAir: Signal for the cylinder temperature sensor is invalid
Short answer
On a Glow-worm heat pump, I.201 means: Signal for the cylinder temperature sensor is invalid. 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: Signal for the cylinder temperature sensor is invalid.
Part involved: Signal for the cylinder temperature sensor is invalid.
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: clears when condition resolved / engineer resets.
Manual reset: engineer resets (Installer level > Reset > Reset maintenance message).
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 plug contact on PCB and harness; check sensor; replace if required.
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
- GeniaAir Appliance Interface installation and maintenance instructions ↗ · 8000043014_01 · page HPIM 7 8000043014_01 p.29 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026