Not on a focus, but before i had a car that wouldn't start i tried a decent diagnostic tester on and it said the same thing - cannot connect to ecu or something similar. Turned out to be no earth to the engine ecu, hence the car not starting or tester not working. Ran a new cable from battery to chassis to ecu. Everything worked fine again.
but before that i did the same as you, changed battery etc to no avail.