my truck has a dead skip, and no scanner so far (snap on, mac, ford's wds and ids scanners) could figure it out. the only way to find it was to take off the valve covers, check to make sure they are all spitting, and then continued on to unplug one injector at a time with the engine running. when you unplug the dead one, nothing will change (rpm/rough idle). when you unplug a good one it should get pissed off.