Voltage constantly high

[OldGuy96]

When I first start my truck (21 GMC Canyon AT4), the voltage indication is around 12 volts, but climbs to around 14.9 to 15.1 within a minute and doesn't decline to "normal" (around 13.4v).

 

I thought the voltage regulator was controlled by the ECU. Is it a separate component that I need to replace, or is there another cause for this high voltage?

[swathdiver]

Your alternator is trying to keep your battery alive.  If you make lots of short runs, your battery has become discharged and either put it on a tender or take a long drive and let it get charged up.

 

If this goes on for a few days, your battery is probably not holding a charge anymore.

 

You can check the battery's state of charge by measuring the voltage at the battery in the morning after the car has set overnight, take note of the ambient temperature and look up a chart to see what the percentage is.  If it is below 65%, it's due for replacement.