Patrick Mahomes looked up at the scoreboard late in the third quarter and was stunned to see the Kansas City Chiefs had taken a 31-0 lead over the AFC West rival Oakland Raiders.

That’s because the Chiefs’ high-powered offense didn’t need to do a whole lot Sunday.

Their defense and the inept Raiders took care of the rest.

Mahomes did throw for a touchdown and run for a touchdown, but it was Juan Thornhill’s pick-6 and a long list of blunders by Derek Carr and Co. that really made the difference.

They allowed Kansas City to cruise to a 40-9 victory and into a two-game lead over the Raiders in the division race.

LeSean McCoy and Darwin Thompson also had TD runs for the Chiefs (8-4), who can clinch a fourth straight division title with a win over New England and an Oakland loss to Tennessee next weekend.

Oakland (6-6) sure didn’t play like a team fighting for a piece of the division lead.

Carr dropped to 0-6 at Arrowhead Stadium with another miserable outing, throwing for 222 yards with a touchdown and two interceptions. The Raiders failed to score twice in the red zone, were penalized 12 times for 99 yards, and watched Kansas City put together a 9½-minute drive almost entirely on the ground that soaked up most of the fourth quarter.

When the Raiders finally scored a touchdown in the last minute, the Chiefs blocked the extra point and returned it for two points — one more insult to Oakland in a game full of them.

It was the Raiders’ seventh straight loss in Kansas City, and the continuation of a major late-season spiral. The blowout came on the heels of a 34-3 loss to the Jets last weekend.

Josh Jacobs was Oakland’s lone bright spot, running for 104 yards on 17 carries.

Meanwhile, the Chiefs were not penalized once in a game for the first time since Dec. 8, 1974.