Armstrong qualified third and made quick work of Team Penske’s David Malukas to occupy second in the opening stanza.
After a drive-through penalty for speeding in the pit lane on Lap 29 relegated Alex Palou down the order, Armstrong went to the lead and controlled the contest until his untimely engine failure.
The Meyer Shank Racing driver led McLaren’s Christian Lundgaard to the tune of two-and-a-half seconds when Armstrong began to slow just shy of the finish.
As smoke began to pour from his car, Armstrong pulled off at Turn 5 and fell to 24th.
“It was all smooth sailing,” said Armstrong.
“I came out of Turn 6 and the engine just started spluttering like it was out of fuel, but clearly it wasn’t, and then it just completely died.
Secure your spot today.
“I don’t know. We’ll have to speak to Honda, see what the issue was. There was no indication that there was something wrong.
“I’m massively proud of the guys and girls on the #66 because we had the quickest car out there today.
“I think we had the quickest car this weekend. Obviously Alex (Palou) was quick too.
“My engineers did everything right, my pit crew as well. Every pit stop was spot on and I thought that was ours to lose.
“I had some time in the pocket ready for Lundgaard. Obviously he was coming, but yeah, just gutted really.”
Lundgaard survived a late caution to take the win. It was a dramatic day for the McLaren driver, who had his own issues at the start of the race when he got into the back of Scott Dixon.
The McLaren driver, who started 13th, went off at the first turn with front wing damage and was forced to pit at the end of the first lap.
Lungaard was given a reprieve on Lap 15 when Romain Grosjean lost his right rear tyre in Turn 5 and brought out the caution.
Running off-strategy, the Danish driver was able to run longer than his rivals and overcut his way in the final phase of the race to shoot to third on the final pit stop sequence.
Lundgaard battled Malukas for second and with fresher tyres eventually dispatched him for second before Armstrong’s engine cried no more.
Results: IndyCar Series XPEL Grand Prix at Road America
| Pos | Num | Driver | Team | Laps/Diff | Gap |
| 1 | 7 | Christian Lundgaard | Arrow McLaren | 55 laps | |
| 2 | 12 | David Malukas | Team Penske | 0.624 | 0.624 |
| 3 | 26 | Will Power | Andretti Global | 1.557 | 0.933 |
| 4 | 8 | Kyffin Simpson | Chip Ganassi Racing | 2.310 | 0.753 |
| 5 | 10 | Alex Palou | Chip Ganassi Racing | 2.968 | 0.658 |
| 6 | 20 | Alexander Rossi | Ed Carpenter Racing | 4.582 | 1.615 |
| 7 | 3 | Scott McLaughlin | Team Penske | 4.641 | 0.059 |
| 8 | 60 | Felix Rosenqvist | Meyer Shank Racing | 5.439 | 0.798 |
| 9 | 14 | Santino Ferrucci | A.J. Foyt Enterprises | 7.452 | 2.013 |
| 10 | 27 | Kyle Kirkwood | Andretti Global | 7.858 | 0.406 |
| 11 | 9 | Scott Dixon | Chip Ganassi Racing | 8.356 | 0.498 |
| 12 | 5 | Pato O’Ward | Arrow McLaren | 9.091 | 0.735 |
| 13 | 28 | Marcus Ericsson | Andretti Global | 10.019 | 0.928 |
| 14 | 45 | Louis Foster | Rahal Letterman Lanigan Racing | 10.819 | 0.800 |
| 15 | 18 | Romain Grosjean | Dale Coyne Racing | 11.658 | 0.839 |
| 16 | 4 | Caio Collet | A.J. Foyt Enterprises | 12.121 | 0.462 |
| 17 | 47 | Mick Schumacher | Rahal Letterman Lanigan Racing | 12.352 | 0.232 |
| 18 | 76 | Rinus VeeKay | Juncos Racing | 13.141 | 0.788 |
| 19 | 77 | Sting Ray Robb | Juncos Racing | 13.750 | 0.609 |
| 20 | 19 | Dennis Hauger | Dale Coyne Racing | 14.666 | 0.916 |
| 21 | 6 | Nolan Siegel | Arrow McLaren | 19.807 | 5.140 |
| 22 | 2 | Josef Newgarden | Team Penske | 36.776 | 16.969 |
| 23 | 15 | Graham Rahal | Rahal Letterman Lanigan Racing | 1 lap | 1 lap |
| 24 | 66 | Marcus Armstrong | Meyer Shank Racing | 3 laps | 2 laps |
| 25 | 21 | Christian Rasmussen | Ed Carpenter Racing | 27 laps | 24 laps |
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