Vaillant ecoTEC plus VU Series Installation And Maintenance Instructions Manual page 45

Fault code Meaning
F.83 Fault: Dry fire
F.84 Fault: Flow/return temperature sensor
F.85 Fault: Temperature sensor
F.86 Fault: Underfloor heating contact
F.87 Fault: Electrodes
F.88 Fault: Gas valve assembly
F.97 Fault: Main PCB self-test failed
Connection No communication between the main
PCB and the user interface
0020238195_07 ecoTEC plus Installation and maintenance instructions
Possible cause
When the burner starts, the temperature change registered at the flow or
return temperature sensor is non-existent or too small: Insufficient water
in the product, the flow or return temperature sensor is not in the correct
position on the pipe
Values not consistent, difference < -6 K
Flow and return temperature sensors signalling implausible values: Flow and
return temperature sensors have been inverted, flow and return temperature
sensors have not been correctly installed
The flow and/or return temperature sensors have been installed on the same
pipe/incorrect pipe
Temperature sensor not connected or is connected incorrectly
Underfloor heating contact open, sensor disconnected or defective
Electrodes not connected or they are connected incorrectly, short circuit in
the cable harness
Gas valve assembly not connected or it is connected incorrectly, short circuit
in the cable harness
Main PCB defective
Electronics defective
45