F.519 on a Vaillant heat pump: Sensor fault: Building circuit return temp
Short answer
On a Vaillant heat pump, F.519 means: Sensor fault: Building circuit return temp. 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 · 4 sources
What it means
On the aroTHERM plus (VWL ../6 A)
The manual says: Sensor fault: Building circuit return temp.
Part involved: Building circuit return temp.
Likely causes:
- The sensor is not connected or the sensor input has short-circuited.
On the aroTHERM
The manual says: Sensor fault: Return temperature.
Part involved: Return temperature.
Likely causes:
- Sensor defective or incorrectly connected to the main PCB.
On the flexoTHERM exclusive (VWF ../4)
The manual says: Sensor fault: Building circuit return temp.
Part involved: Building circuit return temp.
Likely causes:
- Sensor not connected or sensor input has short-circuited.
On the geoTHERM mini (VWS 30/4, VWS 36/4.1)
The manual says: Sensor fault: Building circuit return temp.
Part involved: Building circuit return temp.
Likely causes:
- Sensor not connected or sensor input has short-circuited (shared table row).
Is it safe?
| Does the heat pump stop? | Yes |
|---|---|
| Severity | Fault — 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). 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). 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).
On the flexoTHERM exclusive (VWF ../4)
- Note the full code and text shown on the display (and the date/time it appeared). 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).
On the geoTHERM mini (VWS 30/4, VWS 36/4.1)
- Note the full code and text shown on the display (and the date/time it appeared). 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 and, if required, replace the sensor; Replace the wiring harness.
Who to ask for: Vaillant-trained heat pump installer/engineer (MCS-certified installer).
Certification: MCS installer certification; Vaillant product training recommended.
On the aroTHERM
- Check sensor, plug on main PCB and wiring; replace sensor if out of tolerance.
Who to ask for: Vaillant-trained heat pump installer/engineer (MCS-certified installer).
Certification: MCS installer certification; Vaillant product training recommended.
On the flexoTHERM exclusive (VWF ../4)
- Check the sensor, and replace if necessary; Replace the cable harness.
Who to ask for: Vaillant-trained heat pump installer/engineer (MCS-certified installer).
Certification: MCS installer certification; Vaillant product training recommended.
On the geoTHERM mini (VWS 30/4, VWS 36/4.1)
- Check and, if required, replace the sensor (resistance vs sensor characteristic values in the appendix); Replace the cable harness.
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
15 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
- No heating on the aroTHERM: 2 reports
Sources
- Heat pump control interface module (VWZ AI) operating and installation instructions ↗ · 0020291573_01 · page VWZ AI appliance interface 0020291573_01 p.34; uniTOWER VIH QW 190/6 E 0020291525_05 p.60; aroTHERM split hydraulic station VWL ../5 IS 0020257319_05 p.57 · Reviewed 23 September 2026
- Installation instructions aroTHERM VWL 55/3 A 230 V, VWL 85/3 A 230 V (GB) ↗ · 0020217827_03 · page VWL ../3 A 0020217827_03 p.32; VWL ../2 A 0020147164_09 p.34 · Reviewed 23 September 2026
- flexoTHERM exclusive VWF 57/4 - 197/4 400V installation and maintenance instructions ↗ · 0020213394_05 · page flexoTHERM 400 V 0020213394_05 p.51; flexoTHERM 230 V 0020213395_04 p.45 · Reviewed 23 September 2026
- geoTHERM mini VWS 36/4.1 230 V installation manual ↗ · 0020250181_00 · page geoTHERM mini 0020250181_00 p.36 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026