Vaillant con fault codewhat it means and how to fix it
The display has lost contact with the main board. "con" on a Vaillant is a communication fault between the user interface and the PCB in the electronics box, not a fault in the heating itself.
Take care
Get a Gas Safe registered engineer to look at your Vaillant con
We do not pass enquiries to a repair firm, so there is no form here. Find a Gas Safe registered engineer through the Gas Safe Register, and ask to see their registration card when they arrive — it is the register, not the van, that tells you they are qualified.
What con means
Not every code on a Vaillant display starts with a letter and a number. "con" — short for connection — appears when the display module cannot talk to the main printed circuit board behind it. The 2005 and 2009 ecoTEC servicing instructions list it beneath the F-codes as "con: no communication to mainboard — connection error display mainboard". The ecoTEC plus ..6/5-5 and pro tables describe the same row as a "communication fault between the display and the PCB in the electronics box" and give one check: the cable and plug between the display and the PCB.
The open-vent ecoTEC plus prints it as "con — no communication with the PCB", and the ecoFIT pure and sustain spell the word out, "Connection — no communication between the main PCB and the user interface — electronics defective". What the reader sees depends on the generation, but the cause list has two entries everywhere: the ribbon or plug between the two boards, and one of the boards themselves.
Because the display is the part that would show you a fault, "con" is often what appears immediately after the front panel or the display has been disturbed — a service, a knock, or a board replacement where the display connector was not pushed home. The 2022 CS/1-5, whose display is a different design, does not show "con"; its version is F.707, "no communication is possible between the display and the PCB", with the same three steps — check the connection, replace the cable if required, then the display or the PCB — so a CS/1-5 owner searching F.707 is on the right page.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Display plug or cable not seated | Vaillant's only check on the ecoTEC plus and pro: the cable and plug between the display and the PCB. Disturbed when the front panel comes off for a service or a repair. | Common |
| Display module failed | The user interface board has stopped answering. Replaced as a unit; Vaillant's "electronics defective" on the ecoFIT covers it. | Occasional |
| Main PCB interface failed | The communication port on the main board. Reached by elimination once the display and cable have been swapped or tested. | Occasional |
| Moisture on the electronics | A leak from above onto the electronics box corrodes the low-voltage connectors between the boards. An engineer looks for the leak before fitting parts. | Rare |
Can you fix it yourself?
Stop — this needs an engineer
What it costs to fix
Typical repair cost
£80–£300
Time on site
45 minutes
Reseating the display plug is a call-out of £80 to £120. A display module or main board is £200 to £300 fitted.
Typical UK prices including parts and labour, updated September 2026. Your quote may differ.
Parts you may need
We may earn a commission if you buy a part through one of these links. It does not change what you pay, and it does not affect which parts we list.
Vaillant main PCB (electronics box)
£160–£340Authorised Vaillant parts distributor
Price checked 28 September 2026
Vaillant wiring harness (plug-and-cable set to a sensor or actuator)
£20–£65Authorised Vaillant parts distributor
Price checked 28 September 2026
Own this appliance and had this code? . Nothing is published until a person has read it.
Related Vaillant fault codes
- F.49Get it checkedThe voltage on the eBUS, the two-wire link between the boiler and its Vaillant controls, has dropped too low. Usually a short or a wiring clash on that bus, not a boiler fault.Related: controls and communication
- S.39Get it checkedA protective wait, not a lockout: the "burner off" contact has opened — an underfloor thermostat or a condensate pump alarm wired to the boiler — and heating is blocked until it closes again.Related: controls and communication
- F.78Get it checkedA wiring-configuration code on UK combis: the external control's link box is connected but the hot water sensor input has not been bridged. Vaillant say the boiler itself has not malfunctioned.Related: controls and communication
- S.42Get it checkedA protective wait, not a lockout: an external device — a flue non-return flap through the VR40 module, or a condensate pump — is telling the boiler not to fire. Heating waits until it clears.Related: controls and communication
- F.90Get it checkedCommunication with the actoSTOR hot water module — or on the 2022 CS/1-5 the internal cylinder — has been interrupted. A wiring fault on the link, or a setting left on after the module went.Related: controls and communication
- F.86Get it checkedAn ecoFIT code: the underfloor heating contact is open. A floor-protection thermostat wired to the boiler has tripped, or its sensor is disconnected, and heating is blocked until it closes.Related: controls and communication
Own a Vaillant boiler? Save it and its codes come first next time.
Related guides
Common questions about Vaillant con
Is the boiler still heating with "con" showing?
It may be. The fault is between the display and the board, and the main board can keep running the burner while the display has lost touch with it. Do not rely on that; without the display you cannot see any other fault the boiler raises, so treat it as needing an engineer.
Can I reset it?
The reset key is on the display, and the display is the part that has lost contact, so a press may do nothing. Switching the boiler off at its isolator for a minute and back on is the only householder action, and if "con" returns the connection is genuinely faulty.
It appeared straight after a service. Related?
Very likely. The display connector is disturbed whenever the front panel is removed, and a plug not fully home is the common finding. Ask the servicing firm back.
Technical review
Not reviewed by an engineer
No engineer has read this page. Severity follows what the manufacturer publishes, and we would rather tell you that than let a byline imply a sign-off.
Sources and how we verified this
We establish the facts from the sources below and then write the explanation ourselves. We do not copy manufacturer wording and we do not host their manuals.
- Manufacturer technical and service documentation
This page has not been reviewed by an engineer, and we would rather tell you that than imply otherwise. The severity rating follows what the manufacturer publishes: where they say isolate the appliance and call someone, so do we.