While working in the A&MC box, it may be worthwhile to check whether the FOUL circuit is wired correctly.
In many installations, a single 'figure 8' cable has been used to carry the foul signal for both left and right lanes, with the house earth being used for common.
This is potentially unsafe and should be rectified immediately.
Both left and right lanes should have a 'figure 8' cable each and these should be connected to terminals 7 and 8 inside the A&MC box. There should be no common connection between the two lanes.