Seat Ibiza 6J 1.2 12V does not start, workshop does not find errors

Moin dear community, in my 2009er the Seat contract workshop was unfortunately unsuccessful and could not find the error. He has sparks, 13 bar compression and free injection nozzles, 3 new coils and candles, all sensors tested, control times checked, wire harness probably so far inconspicuously well and yet all 3 candles are completely black and worn out after a few start attempts. The seat workshop is also at a loss and would next be the harness and the engine control unit on gu t Renew happiness, but can it be broken at all if there is an ignition spark on all 3 cylinders? In the meantime, the car is in a free workshop and they also don’t get any further. By the way, no error is stored in the error memory.