42 on a Vaillant geoTHERM: Error HP return sensor T5 (short circuit)
Short answer
On a Vaillant heat pump, 42 means: Error HP return sensor T5 (short circuit). The heat pump stops until the fault clears or is reset. An engineer is needed to fix the cause.
Reviewed 23 September 2026 · 1 source
What it means
The manual says: Error HP return sensor T5 (short circuit).
Part involved: Error HP return sensor T5.
When it shows: Fault-induced shutdown: heat pump shut down until the error is reset (Menu I1).
Likely causes:
- Heating return temperature sensor defective or not properly plugged into the PCB.
- Emergency mode (electric auxiliary heating): possible.
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.
Manual reset: yes - by the engineer, by resetting the error in Menu I1 (error history) after removing the cause; emergency mode via the electric auxiliary heater can be enabled where marked.
What you can check
- Note the full code and text shown on the display (and the date/time it appeared). The geoTHERM operating manual says: do not attempt to remove the source of the malfunction yourself; only an expert technician can remedy the fault and reset the error code. Warnings (26, 36) and temporary faults can be noted and discussed at the next inspection.
What the engineer will check
- Check PCB plug and harness, check sensor, replace 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
1 owner and engineer report mentions this.
- No heating on the geoTHERM (VWS/VWW ../2, VWS ../3): 1 report
Sources
- Installation instructions geoTHERM VWS 6-10kW 230V (GB 08/2008) ↗ · 0020046310_01 · page geoTHERM VWS 6-10 kW 230 V 0020046310_01 p.74; geoTHERM VWS/VWW 6-17 kW 400 V 0020057444_01 p.72; geoTHERM VWS/VWW 22-46 kW 0020051574_04 p.103; geoTHERM exclusive operating manual 0020071929_00 p.27 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026