Hi.
The people repairing your ECU will probably have told you already but make sure you replace the coil and leads with genuine Ford units and the plugs when you re-fit it. The coil may well be what damaged the ecu in the first place. If you don't you may find any warranty given by whoever is repairing the ECU will be void!

I'm told that a faulty ECU can damage a coil pack and a faulty coil can damage the ECU!!

Also - look for evidence of water ingress into the plug gallery. This may be what caused the problem in the first place.

Lastly - advise the customer that, if they have been driving with a mis-fire, there is a possibility of damage to the cat. If you don't cover your back you will be amazed how quickly they'll come back with the old "ever since you did the coil.... etc"!

Regards - Paul.

(P.S. - I'm awaiting the return of my third Focus/Fiesta ECU from the repairers!)