my friend has a 99 civic,and is turning off while driving.i dont know much about civics,most of my knowledge is on legends.I already changed his main relay about a year ago.i was thinking his alternator.his bat. is not dying,and the car starts right up ...last time it shut off was on the interstate.at about 55 mph...fuel pump?...alternator?...what else?

 

Welcome in,it's very possibly the cam position sensor,or an Alternator.

 

If there is no check engine lights, and everything shuts down when the car dies, check somewhere for a loose battery connection.

 

I searched this forum last week, and found a youtube video about this same problem, 99 civic, stalls, he changed the ignition switch. Well, I can't believe the difference, car starts first time always, and hasn't stalled. The hard part is disconnecting the terminals from the large block with 7 wires, I couldn't figure it out, I tried for 40 mins. I took a hacksaw to it. If you need a link to the video, post a message and I'll forward it to you. I changed the cap/rotor and fuel filter. Good luck.