Moments after putting the No. 01 Chip Ganassi Racing Cadillac up front by passing Alexander Rossi in the No. 10 Acura, Kevin Magnussen was penalized for wheel rotation while on the jacks during a pit stop that occurred minutes earlier under caution. The leader served a stop-and-go penalty, dropping Magnussen to fourth.

A major development occurred 22 minutes into Hour 19 when Felipe Nasr took the No. 31 Whelen Engineering Cadillac DPi-V.R to the garage. The pole-winning car was stuck in third gear. Nasr had led the opening 20 laps of the race and was running third at the time of the incident.

“There were two different things, and I don’t think they were connected,” Nasr said. “We had an exhaust issue, that’s why the engine was sounding a little weird, but we kept going and the car had pace. Then unfortunately as I was leaving Turn 6 on the transition to the banking I lost the drive going to fourth. I heard a pretty loud bang on the gearbox. I feel terrible for everyone — we put in good work all weekend, last weekend as well. We need to come back with better luck next year. That’s racing. This is the hardest race we go to, the biggest race of the year, and it caught us off guard this year. This is still the race I love coming to. It is what it is. It’s a long season. It was not our turn today.”

One of the lead quartet, the No. 60 Meyer Shank Acura, was also working to find pace while running third.

“Something seems to be amiss with our car as we’ve lost a little bit of pace,” said Dane Cameron after getting out of the No. 60 Acura. “Hopefully, if we get a yellow we might give something a shot to find a little bit more speed. We’re expecting it to be pretty warm for the end of the race. We’re still in the mix, and there’s a fair amount of time left. This car didn’t have the balance, to be honest. We’re making mechanical changes, aero changes, trying to get some speed. We haven’t really gone any faster than we did at the beginning, but the balance is a lot more consistent. Our car’s getting a little bit better. I think ultimately it’s going to come down to how much the temperature is going to rise at the finish.”

Caution waved for debris on the front straight midway through the hour, shortly after Nasr went to the garage. The 18-minute delay left the top four bunched for the restart. Rossi led in the No. 10 Konica Minolta Acura, although Magnussen made the pass to put the No. 01 Chip Ganassi Cadillac back up front — moments before his penalty was announced.

At the end of the hour, Rossi led Simon Pagenaud in the No. 48 Ally Cadillac by 0.864s, with Montoya two seconds in arrears. Magnussen was fourth, trailing by 37.297s.

There was no change in GTLM, with Nick Tandy (No. 4) leading Jordan Taylor (No. 3) as Corvette remains 1-2, followed by Jesse Krohn in the black No. 24 BMW and Jules Gounon in the No. 62 Ferrari.

Ryan Dalziel took over the LMP2 lead in the No. 18 Era Motorsports ORECA, closely followed by Christopher Mies in the No. 82 DragonSpeed USA ORECA and Tim Buret in the No. 8 ORECA.

The battle for the GTD lead continues, with Philip Ellis taking the lead in the No. 57 Mercedes followed by Niclas Nielsen in the No. 21 Ferrari. Five cars remain on the lead lap.

Scott Andrews continues to lead LMP3 by three laps in the No. 74 Riley Motorsports Ligier.