BoilerFaultCodesVaillant con Fault Code
Also written as CON — same fault.
The Vaillant con fault code means: your boiler's internal computer screen and main control board are having trouble talking to each other, meaning the unit cannot function.. It appears on 22 Vaillant models in our database.
Technical description: no communication to mainboard | connection error display mainboard
What causes the Vaillant con fault?
This fault occurs when the communication link between the boiler's control interface and its main brain is broken, preventing any commands from being processed. The most common reason is a loose or damaged internal wiring connection, though it can also be triggered by a power surge that has damaged the delicate circuitry on one of the electronic boards. Without this digital conversation, the boiler cannot safely ignite or operate.
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
Find a Gas Safe engineer
Search the official register for a qualified engineer in your area.
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A cover plan would handle faults like con: no call-out fees, no surprise bills.
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Vaillant models that show the con fault code
Click a model for the full fault code list and model-specific repair notes.
- ecoFIT Pure 412 Repair: £200-450
- ecoFIT Pure 418 Repair: £200-450
- ecoFIT Pure 425 Repair: £200-450
- ecoFIT Pure 435 Repair: £200-450
- ecoFIT Pure 825 Repair: £200-450
- ecoFIT Pure 830 Repair: £200-450
- ecoFIT Pure 835 Repair: £200-450
- ecoTEC Repair: £150-£450
- ecoTEC exclusive Repair: £180-450
- ecoTEC plus Repair: £180-450
- ecoTEC Plus 615 Repair: £180-450
- ecoTEC Plus 620 Repair: £180-450
- ecoTEC Plus 625 Repair: £180-450
- ecoTEC Plus 630 Repair: £180-450
- ecoTEC Plus 635 Repair: £180-450
- ecoTEC Plus 832 Repair: £180-450
- ecoTEC Plus 836 Repair: £180-450
- ecoTEC Plus 840 Repair: £180-450
- ecoTEC Plus 938 Repair: £180-450
- ecoTEC Pro 30 Repair: £200-450
- ecoTEC sustain Repair: £180-450
- Home System Repair: £180-450
Source
Vaillant con fault code information on this page is sourced and cross-checked against the official Vaillant service / installation manuals for each affected model.
- Vaillant ecoFIT Pure 412 manual (PDF)
- Vaillant ecoFIT Pure 835 manual (PDF)
- Vaillant ecoTEC manual (PDF)
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 7 July 2026