The fixed wheelbase of a 143 means that it's not allowed on the Southern Region of Network Rail - said to be because of all the third rail they've got in the area, but that's possibly just an excuse for Warminster to Salisbury.
Blanket ban on all Pacer classes on the former
SR‡. There was a special exemption granted for the 143's to run
ECS▸ to Eastleigh over a single set route for the refresh programme to take place. Apparently the axlebox / primary spring yokes & the metal front valance / battery cases (which are resiliently mounted on the underframe) on 143's get rather too close for comfort to the third rail when running in loaded condition. 142's obviously not cleared, there is no reason (or qualified
FGW▸ crew) for them to run on the former SR.