AFCI trips immediately
There is an easy way to find out if your AFCI is bad. Disconnect the circuit but not the neutral from the breaker and see if it resets. If it does reset your AFCI is working and somewhere in your circuit you are getting an arc. Exactly where can take some work. 

Employ the normal troubleshooting method of disconnecting half of the devices and see if the AFCI trips. The you take half of the half and try it again. Whether the AFCI trips or not tells you which direction to trace. No trip you add to the circuit. Yes trip you work toward the breaker. Eventually you will find the problem device. If that's a little hard to digest then just start at one end of the circuit and begin disconnecting fixtures one at a time until you find the problem.

Good luck.