ok so i threw on some headers no o2 sensor pluged in but my alt will not hold 14 volt charge after i put the headers on so i checked the voltage readeing right now it is 12.67 volts give or take but on a hot day in sacramento it runs fine but at red lights the spedo goes nuts and my lights all dim up n down alot and i took it autozone or something they told me it was the alt changed it out 3x and still the same any ideas becouse im confused need help plz and thnk you guys

 

Holy run on sentence Batman!

First off, header. Singular. You only have one. Second off, your header and O2 sensor have nothing to do with your alternator. Finally, check your grounds to make sure they're all still there, clean them up, reattach whatever ground you took off, and post whatever CEL codes you (probably) have.