F.103 on a Vaillant heat pump: Fault: Spare part ident
Short answer
On a Vaillant heat pump, F.103 means: Fault: Spare part ident. 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 · 2 sources
What it means
On the aroTHERM plus (VWL ../6 A)
The manual says: Fault: Spare part ident.
Part involved: Spare part ident.
Likely causes:
- Incorrect control PCB installed on the outdoor unit.
On the aroTHERM
The manual says: Fault: Spare part detection.
Part involved: Spare part detection.
Likely causes:
- Parameter 'Max. duration power supply cut-off' poorly set; replacement main PCB or frequency converter does not match the product.
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). Check the heat pump supply breaker/isolator (and back-up heater breaker, if separate) at the consumer unit is on; if a breaker has tripped more than once, leave it off and call an engineer - do not open any casing. 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). Check the heat pump supply breaker/isolator (and back-up heater breaker, if separate) at the consumer unit is on; if a breaker has tripped more than once, leave it off and call an engineer - do not open any casing. 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)
- Installing the correct PCB.
Who to ask for: Heat pump/controls installer familiar with Vaillant eBUS controls.
Certification: MCS installer certification; Vaillant product training recommended.
On the aroTHERM
- Diagnose per cause list in the aroTHERM installer fault table; inform Vaillant technical support (0330 100 3540) for refrigerant-circuit faults.
Who to ask for: Heat pump/controls installer familiar with Vaillant eBUS controls.
Certification: MCS installer certification; Vaillant product training recommended.
What you might notice
What owners and engineers report
22 owner and engineer reports mention this.
- Controller / thermostat problem on the aroTHERM plus (VWL ../6 A): 12 reports
- No heating on the aroTHERM plus (VWL ../6 A): 8 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.59 · 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
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026