|each cylinder pack to fire. The PTU (power transistor unit) uses that signal to activate a high current driver that actually causes the pack to spark. Back in the day the technology wasn't able to have the PTU part of the ECU, so it was done as a separate part.
So what you are troubleshooting is whether or not the PTU is getting a firing signal from the ECU. ECU could have an issue or more likely the wire run from the ECU to PTU - if this is the issue. The other candidates would be the PTU connectors and wires or the wiring run from PTU to coil pack. As I recall the coil pack connector is also supposed to have a +12v line - if that isn't getting voltage things won't work either. It is also totally possible the PTU has failed but that doesn't happen much...
Just break the pieces down into bite size chunks and check each with a digital meter. Make sure the ECU to PTU Connection is good, coil pack has voltage, etc. My bet is you've got a PTU harness line or connector issue. Some just cut out the PTU replacement adapter harness all together and directly connected the car's harness to the new style PTU plug. Its a bit of work but gets rid of redundant wiring and connectors. I never needed to do it and when I relocated my PTU to under the front nose panel the extra cable length was helpful to have.
Did this whole spark issue start out of the blue or was some work done on the car (i.e., T-belt servicing)?