Have a 2011 GMC 1500 V8 5.3. Just put a new battery and alternator on it but it still is not charging. I had O'Reilly'* test it and it came back with a P0621 code on it which is something about the generator. Had the alternator checked, turned out fine but it is not charging the battery. Any suggestions?

do yo have a service charging system light illuminated? (little red battery)

assuming you do because of the code, check the wiring harness leading from the alternator. the harness may be chaffing against the A/C compressor line.

yes - the light is on. I will definitely check the harness. Thank you.

Well - checked wires / harnesses - even checked the grounds - doesn't seem to be those things. Any ideas on what else it could be would be greatly appreciated.

If the alternator was tested ok, and you have power on the large red wire into the Alternator, then if it is not the harness or connections in the harness, you could have a bad ECM.....really need a scan tool, like Tech II, to diagnose this.....

Do any of the parts stores have a Tech II to help diagnose this issue or is it something that I have to go to the dealership for? Thank you for your help also.

Most parts stores do not have high end scanners to read all codes, and all data....

The BCM, ECM, and IPC all communicate via a data line......you probably need to read "U" codes(communication), as well as "P" codes...most only read "P" codes and Powertrain data.....

Basically the ECM monitors alternator output, and when it needs to charge, it sends a 5 volt signal to the alternator, which turns on the voltage regulator which controls alternator output......that'* why you have to check the F and L connections at the alternator that they are tight.....back probe the F to get alternator output, and back probe the L to check for 5 volts.....

You may have to back probe the same line connections at the ECM, to check for an open circuit in either line....

Makes sense - I will check both lines with my meter and see if I am getting 5 volts. Hopefully it is just a connection. Otherwise, I may just have to break down and take it to someone. Thank you for your help.