Ill post the codes and then my issue and see of anyone can help me get over this nightmare 🤦🏽‍♂️

codes:
pcm- p1431 H
pcm-p1518 H
tcs-c1214 H C (I already knew about this one)
tcs-c1278 H
*thats it*

problem: theres a few my car doesnt show me the correct volts when KEY ON ENGINE OFF-11.8,KEY ON ENGINE ON-12.8-13.5 BATTERY COMPLETELY OFF CAR- 12.4, recently i got a “reduce engine power” And the car “cut off” and my engine light came on but my dumb-self didn't check the codes at the time i just turned it off waited a few seconds up to a minute turned it on and it drove like nothing was wrong and engine light was gone. And thats where im at right now car doesn't get the right volts and my car recently “cut off” with that message. I already put a new alternator the battery is good i checked and all the terminals/grounds are good. So im thinking on trying the ignition switch but not sure please let me know

 

How old is your battery?

 

I would check your battery connections first and then depending on the age of the battery possibly replace it. You can also put it on a charger first and see if the problem disappears. I always keep my C5 on a battery tender as a low voltage condition will throw all sorts of errors. Most of the codes you are seeing have an H behind them and are not current, The one that is current (C) and has history (H) is for your ABS electronic brake traction control module.

 

got it on January of this year the battery is good I already had it checked

 

Too late at night when I replied the first time - need to read the post more closely. What year is the C5?

 

2001

 

I still think the pcm-p1518 H code is what set the tcs-c1278 H which threw the reduced engine power. A problem with the TAC will throw the P1518 code and if severe enough will shut the car down. This should only happen if more codes are set at the same time related to the TAC. In your case it appears there were no other related codes set. BTW - if a code occurs it will remain in history (H) until 40 starts where the condition does not reoccur or it is cleared manually. So turning the car off and on would not have cleared the codes. There were problems with the 97 and 98 C5s with the throttle actuator control (TAC) module and replacing it fixed the issue. Yours is a 2001 so maybe not the issue although there are other posts on newer C5s such as yours where replacement of the TAC module, or just the throttle position sensor, also fixed the problem. A couple of times the issue was due to the owner giving the engine bay a bath and got the connections to the throttle body wet. There are also posts indicating the ignition switch was the issue and replacing it solved the problem so I would not count it out. I would reset all of the codes and see if it happens again and then check the codes to see what pops up. I lean toward a problem with the TAC but am not able to definitively prove that.

I would disconnect the electrical connector to the TAC and check the electrical contacts for corrosion. If no corrosion is visible, I would plug the connector in and out a couple times to let the electrical contacts surfaces make better contact if needed. Sometimes that is all it takes.

BTW - I think the voltages you are seeing appear to be correct. Car not running/switch on will produce a lower voltage as you impose an electrical load. Once started the alternator will boost the voltage. A disconnected battery will float a little higher with no load but will be less than when the car is running.