. An announcement was made that there was an engine fault and we returned to the gate where a "procedure' was carried out, which then entailed a 15 minute engine run at high power, so we left the gate to do that. It didn't cure the fault so we returned to the gate.
That's a common problem with engine faults - they generally don't show until
after you start the engines. Further, ETOPS means a more restrictive MEL (on the 767 there were different columns in the Master MEL for ETOPS and non-ETOPS - since the 787 was designed for ETOPS it may always use the more restrictive ETOPS MEL).