The Brazilian started the second race of the doubleheader from the pole alongside championship challenger Scott Dixon and got a jump on the standing start to take the lead. But Dixon was screaming on his radio within minutes that Castroneves' car was leaking oil everywhere and splashing onto Dixon's tires and visor.
Castroneves said little to his Penske Racing team, but detected a vibration in his car with every shift of the gears after just a handful of laps. The problem worsened and he came to a complete stop on the course at Reliant Park after just 11 laps.
"It's frustrating and disappointing," Castroneves said. "It hurts. It really hurts."
His car was towed to the garage with a broken gearbox, Castroneves made the long walk back still wearing his helmet, and team owner Roger Penske retreated without comment into a team transporter.
Castroneves eventually returned to the track, 36 laps down, and finished 23rd.
Up 49 points over Dixon when he arrived in Houston, he now goes to the Oct. 19 season finale trailing Dixon by 25.
"Obviously, this is not how we wanted our weekend to go," he said. "I'm not sure if we hit something or what. It was a very helpless feeling. What an incredible job by my guys to fix the damage. We were at least able to get back out on track and pick up another position.
Dixon thought Castroneves damaged his car right after the start when he hit a bump in the track in Turn 1.
"He hit really hard going through Turn 1 kink, instead of sticking to the left there, for some reason he went right in the middle and that's when some of the underwing fell off when he hit that hard," Dixon said. "With the amount of oil that was coming out of that thing, I knew it was pretty terminal."
On Saturday, in the first of two races at Houston, a problem with the gearbox caused Castroneves to finish 18th while Dixon won his fourth race of the season. It allowed Dixon to cut his deficit from 49 points to eight headed into Sunday's race.
As Castroneves watched helplessly while his Penske crew replaced the gearbox, he acknowledged there was little he could do but wait to see where things stood after the race. Dixon needs only to finish fifth or better at Fontana, Calif., to clinch his third title.
"I might collect my own thoughts in my own way at this point," Castroneves said. "The good news is we have a very good car for Fontana."
Castroneves led the standings for 10 races this season and came into Houston as the only driver to complete every lap this year. After having no problems this season, he was hit with two in two days at Houston, where he entered the doubleheader event with a good points lead.
"It doesn't make sense, does it?" team owner Penske asked. "As far as I'm concerned, our guys have done a terrific job. The reliability and durability, that's what it takes. We'll just have to see what happens at the end. Helio's done a great job, and it's a disappointment."
Penske had said earlier the team needed only to race smart and conservatively to get Castroneves, a three-time Indianapolis 500 winner, his first IndyCar Series title. The Penske organization is also seeking its first championship since 2006.
"There is still a lot of racing left. We know we have a strong car in Fontana," Castroneves said. "We were fast in the test there last week and we will definitely be going for it."