This fault code is to do with the coding resistor that goes towards identifying unique models to the PCB. The F42 Fault’s cause is that of a short circuit / interuption in the coding resistor output range that can be located in the cable harness at the heat exchanger or the gas group resistor on the PCB
The coding resistor as shown in Image 1 is part of the harness. The resistor is the little white connected on it’s own, that is usually connected to the harness. As shown in image 2, the coding resistor is the yellow device inserted into the PCB. This version is usually found in the newer Vaillant Ecotec PCBs
F42 Fault Code Repair
It would be a good idea to double check all the connections between the coding resistor and the PCB, using a multimeter if necessary. This fault can occur in combination and possibly due to F70. To address an F70 Fault code see the F70 Fault Code.
Part Nos
0020210202 Cable Harness Ecotec Plus 624, 831, 832