Hi, Aimee and the usual suspects are:
1. Severely discharged or a damaged battery must have 12.5 volts or more and be able to pass a proper "LOAD" test if necessary, you may have a preliminary reading of 12.5 volts or more but little or zero amperage the battery is faulty and must be replaced, AGM batteries fail in this scenario more so than lead-acid batteries.
2. Faulty alternator/generator and or voltage regulator.
3. Loose or corroded battery terminals and or cables especially the "NEGATIVE" cable, look for loose, corroded, or broken connectors inside the cable harness at "BOTH" ends.
4. Faulty main circuit breaker or ignition switch, check for loose connections and continuity.
5. Faulty system relay or ignition relay, check for continuity.
6. Faulty ignition coil, ignition/electronic module.
7. Faulty CKP, CPS, CMP, MAP, TPS, or BAS sensor, corroded, loose or broken wire connector pins/sockets.
8. Throttle cables and or idle speed improperly adjusted hot idle speed should be 950 RPM to 1000 RPM.
9. Faulty neutral, side stand or clutch lever safety switch.
10. Fuel tank empty
11. Fuel tank contaminated with ethanol sludge.
12. Water or dirt in the fuel system, or clogged filter.
13. Quick disconnect check ball stuck.
14. Restricted, blocked or kinked fuel line.
15. Faulty fuel pump.
16. Faulty pressure regulator.
17. Faulty or clogged fuel injectors.
18. Lean angle switch is faulty or needs adjustment.
19. Catastrophic engine failure, perform a compression test.
For more information about your issue and valuable "FREE" downloads that you need for viewing or printing please click on the blue links below. Good luck and have a wonderful day.
APRILIA RS4 125 Service Station Manual