Vaillant Ecotec 624 with VRC 430 showing -50c intermittently

[neil22]

This boiler and its control gear was installed almost two years ago and the guarantee is about to run out! The system seems to work fine except for the weather compensator showing an outside temperature of minus 50 centigrade 6 to 10 times- or more? a day ( usually for about 15 to 20 seconds). The Vaillant engineer has been in four times in this period and changed some of the electrical boards (VR65 control center, VRC693 outside sensor) and original VRC400 was upgraded to the VRC 430 at our cost. Some points to note; whilst the engineer changed the VRC693; the VRC 430 showed -50 degrees continuously ( open circuit). Also when we had the VRC400, the intermittent reading showed -40c which is probably the minimum for that unit. I have had the floorboards up and made sure the connecting cable is not running parallel to any mains cables for any length which could cause a false reading. When the engineer left last time he had to admit that the problem was beyond him . I have just ignored the problem since but am worried as the guarantee runs out in two weeks. My main concern is that this situation is likely to operate the frost stat causing the boiler to fire up when not needed, although I have not observed this condition because of the short duration of the fault. I am looking for a technical explanation to this problem to put to Vaillant, as all I have is a photo of the VRC 430 showing this condition, as it never happened when the engineer was there.