Gas Safe: Gas work, flue work, and PCB replacement on a gas boiler must be carried out by a Gas Safe registered engineer. Homeowners can check obvious items such as heating pressure, condensate overflow, and whether a control is unplugged — then call an engineer.
Always confirm the part number against the boiler GC number and the installation manual before ordering.
Overview
Com (sometimes COM or communication error) means the user interface / display has lost serial communication with the main PCB. There is no F-number. The boiler is usually dead apart from that message.
Vaillant meaning: Communication fault — no communication with the PCB
Possible causes
- Ribbon or interconnect cable between display and PCB unplugged, pinched, or failed
- Display / fascia failed
- Main PCB failed
- Moisture on either board
- After a fascia change, the new display not seated on the connector
Possible solutions
- Isolate mains. Remove the fascia and reseat the display interconnect. Look for green corrosion or a creased ribbon.
- If Com appeared after a panel-off service, the cable was almost certainly not fully home.
- Swap a known-good display only if you can set DSN (F.70) — do not change display and PCB together without coding.
- Replace the PCB if the cable and display are proven.
Possible parts needed
- Display / UI for the exact range
- Display interconnect / ribbon
- Main PCB (and DSN coding — F.70)