I just finished rebuilding the motor on my 04 g35 coupe mt and it drove perfect for the first 10km then would flash battery and brake dash lights at 2000 rpm followed by limp mode at 2500 rpm cutting throttle and flashing abs, slip and vdc lights. I scanned ecu and have no codes showing and battery is showing 12.8 volts with the car off and 14.6 at idle as well as no visible electrical issues, all lights work fine, no flickering, etc. Battery is about 1-2 years old and was recharged before starting the car and alternator is new with the rebuild. Anyone know whats happening??
Edit: went for a drive again drove fine first 10 ish minutes and then started giving me erratic voltage between 14-17 volts if I went over 2000 rpm putting me in limp mode same as before. Voltage stays normal at 14.6 when revved in neutral past 2000rpm. Problem only appears while driving.

 

Clamp some test leads onto the battery and run the cables into the cabin so you can watch voltage when it's actually having the issues.

Probably going to end up being either a lose wire from the installation (double check the primary engine-chassis bonding jumper, the big wire on the right side that goes from the timing chain cover to the chassis under the coolant reservoir), or a lose battery terminal wire, or an intermittently failing alternator.

 

Just went over all my grounds then checked the voltage regulator plug on the alternator (should have been the first place to check) and it was loose. Connected it properly and I'm sure this was my issue although yet to test drive. Will update soon if it is fixed.

 

Problem was indeed the voltage regulator not being properly connected. Can't believe I overlooked this. Regardless, thanks for the insight Cleric.

 

Despite no codes, erratic voltage suggests a potential charging system problem. Check the connections on the alternator, battery, and grounds. Ensure the alternator is correctly wired and the belt tension is optimal.