On my car the ecu can flash out some usful fault codes via the PATS led if It fails to start under certain conditions. If when you turn the key and the led flashes quickly after a while it will then flash out a code. I think it's 11 for a fault on the immobiliser ignition tranciever coil.
Being a sad old git who hates electonics I have a coil in my spares box too. As it contains it's own chip on a pcb it will probably fail at some point. Its a critical item as there is no way the ecu will run the car without it.They can be picked up on ebay for a few quid second hand (they are not coded to the ecu).
