Hi, Craig it should be noted that the reasons your check engine light constant or flashing, will or will not start, turns over or not conditions varies from the year, make, and model and you should always refer to your owners/service manual for proper diagnostic procedures. It should also be noted that any type of prior work or event IE: adding accessories, electrical curiosity/adventures, laying bike down/crashes, rainstorms/bike washings just before EFI light issues started can be significant hints/aids into tracking down the gremlin, also carry the appropriate jumper wire to access fault codes to reduce the risk of being stranded or towed, that being said the usual suspects are:
1. Faulty Fuel Pump, fuse or system relay switch.
2. Battery starting to fail due to old age/damage, perform a load test.
3. Discharged battery, check battery terminals for damage or corrosion, check the battery cables at "BOTH" ends for loose, corroded, or broken connectors, "INSIDE" and outside the cable harness, perform connector wiggle test and check cables with an ohmmeter if necessary.
4. Faulty switches/sensors: run/off, ignition, clutch lever, neutral, side stand, tip over, fuel, and or their connections.
5. Broken wire or worn insulation exposing wire to a ground situation especially inside wire harness at tight bends around fairing brackets, under dash panels, under fuel tanks over cylinder heads etc. Many harnesses are open on the ends that will allow water to enter and accumulate at v-bends.
6. Check for generated fault codes.
Dielectric grease and contact cleaner are your best friends for wire/cable/harness connectors, look for corroded, broken, or loose pins/sockets.
For more information about your issue please visit the websites below. Good luck and have a nice day.
couple of Gilera Nexus questions Thanks Twist Go Magazine
iQ4bike Precise diagnostics for universal motorbike motorcycle
Gilera Nexus 500 Workshop Manual
Genuine Gilera motorcycle parts spares Fowlers Parts
Gilera NEXUS 300 Owner Manual
Btw, I’m available to help over the phone in case u need at https://www.6ya.com/expert/gregg_c0ec1df182c7330e