Vaillant F42 Fault Code

Welcome to the fault-codes section of this site. The following table or page contains the information required to help repair and diagnose the fault codes being shown on the boiler display. You may be guided through the fault finding sequence or steps, for rectifying your boiler with this fault code. Suggested repair solutions may be provided by a link to a repair page. NB Some fault codes are only applicable to a specific range of boilers only.

Vaillant F42 Fault Code

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

Image 1: Cable Harness with Coding Resistor



Image 2 : PCB Coding Resistor


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

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