Ground cart... Turns out we had some pretty lousy ground carts at Boeing Field - we got several complaints during the flight testing that autostart didn't work - turned out the ground carts were so weak that N2 spooled up so slow it was tripping the 'sheared starter shaft' autostart logic
The PW4000 usually requires 2 air carts to meet a healthy n2 for start, might light off a slow almost hung start from min motor with one cart. Diverted once in an MD-11 with 4062's on it, APU out took 30 minuted arguing with the airport authority to allow a hot refuel with #2 running.