98 civic automatic lx,problem im having is first the d4 light flashes,but what im trying to figure out is the check engine light that is on pulsates dim to bright dim to bright if you turn the headlights on you can see the same pulsating in the headlights,i have a constant 12.5v at battery,i have checked fuse 15,checked the harness under the intake manifold,and replaced the fuse box under the hood for the e.l.d what else should i look at?

 

From my experience, you should have at least 13.3v when the car is running. As for the flashing d4, problem with transmission. How's your speedometer ? Does the car drive and shift smooth?

 

Sounds like the alternator may be toast.

 

^ That's where I was going, and for a 1998.... I'd be checking all the grounds and wiring associated with the alternator as well.