M.203 on a Vaillant aroTHERM plus: Connection fault: Disp. not recognised
Short answer
On a Vaillant heat pump, M.203 means: Connection fault: Disp. not recognised. 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: Connection fault: Disp. not recognised.
Part involved: Disp. not recognised.
Likely causes:
- Display defective; Display not connected.
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). 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.
What the engineer will check
- Check the quality of the plug contact on the PCB and on the wiring harness; Replace the display, if required.
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
20 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
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