First thing id check is the new loom has the same part number on the tag as your original loom , id also check the wiring blocks between new and old looms for wiring ie, colour of wires to pin locations on each of the coloured block conectors , having had many days of chaos rewiring an A2 i know there can be several looms for the same model dependent on what year it was made and possibly your new loom is wired differently to your ecu, next if there identical looms and same part number id be checking the relays as sure one feeds power to the ecu when the ignition is turned on and then the two red power leads in the underfloor compartment where the front loom conects to the main loom , check those large fuses are clean on there fingers and where they plug into, theres also a fuse next to the positive terminal on the battery , unsure if this could affect anything but easily missed and worth a look see , theres also a main earth from the battery under the rear carpet bolted against the inner sill , just behind and below where the end of the passengers seat belt bottom bolt bolts to the inner sill on the outside of the carpet for a rough location .. I also check the main power lines red cables on the starter motor are all clean