How to Diagnose a Failed Fuel Pump! Learn how to diagnose a failed relay in this video! So, your horn won't honk, or your headlights won't light. You checked the fuses, and they look fine! What's going on? Time to check the relays! Watch this video to find out how a relay works, what can go wrong with one, and ways you can check yours to see if you need to replace it! #failedfuelpump #autoparts | 1A Auto Parts