From the recall notice filed with the National Highway Traffic Safety Administration, there was a problem with the software driving the taillights. The software itself is embedded in the microcontroller in the taillight assembly.
The software issue is making the taillights partially on or ineffective at all. That extends to brake lights, turn signal lights, backup lights, side markers and clearance lights.
In some cases, this software issue keeps the taillights on even after the vehicle is turned off.
This software issue was discovered by two GM employees based on a Hummer EV field report. GM said it received three complaints about the taillights not working from January 8 to February 7. There are no reports of crashes due to software issues.
A GM spokesperson said it had contacted all owners and replaced the faulty taillights. Other Hummers that haven't shipped are repaired prior to shipping.
“All customer vehicles have been repaired and we apologize for any inconvenience this issue has caused to our customers,” a GM spokesperson said by email.