The No. 912 Wright Motorsports Porsche 911 GT3 R of Dirk Werner is out front of the California 8 Hours at the halfway mark.
Werner, who is due a pit stop, holds a 42-second lead over the second-placed No. 29 Land Motorsport Audi R8 LMS of Kelvin van der Linde, who is also yet to make its fourth stop of the race.
The German took over the lead following a stop for the No. 27 HubAuto Corsa Ferrari 488 GT3 of Miguel Molina, which has led for the most part.
The lead battle, however, was compressed after a slow stop for the Taiwanese Ferrari in the fourth hour, which put Mathieu Jaminet out front in the No. 911 Park Place Motorsports Porsche.
While an air gun issue delayed the Ferrari, trouble also hit both the No. 107 Bentley Continental GT3 and No. 44 Strakka Racing Mercedes-AMG GT3, which have both been behind the wall with mechanical gremlins.
The Bentley lost considerable time in the third hour with a gearbox sensor issue while the Tristan Vautier-driven Mercedes was wheeled to the garage late in the fourth hour with power steering failure.
GT4 continues to be led by the No. 41 PF Racing Ford Mustang GT4, while issues struck the one-and-only GTC contender, the No. 88 ARC Bratislava Lamborghini Huracan Super Trofeo when it briefly stopped on track in the second hour.