I need to know the way to find what's wrong or bad and fix it. The fuel pump was replaced and fuel filter replaced. Now the 543 code comes up and the car won't start.

. Test all under hood and under dash fuses with a test light or multimeter:

https://www.askmehelpdesk.com/cars-trucks/faq-how-troubleshoot-repair-maintain-hondas-selected-other-vehicles-46563-3.html#post252145

We are primarily interested, here, with the fuel pump fuse being good. Be sure to test for power through the fuse. If none, remove the fuse and clean the contacts.

. Remove and clean contacts to the fuel pump relay, located in the under hood fuse/relay box. When you turn the ignition switch to ON, do you hear the fuel pump relay click and the fuel pump run? If the problem persists, you may need to replace the fuel pump relay.