Four hours of “American Idol” carried Fox to a primetime demo victory last week, but the franchise’s declines — along with nothing-special results elsewhere on the network’s sked — have resulted in sizable year-to-year falloffs.

Last week, a pretty clean one with no major sports or awards shows, saw Fox fall 17% below its young-adult performance in the same frame a year ago, according to Nielsen. ABC and CBS matched their deliveries of last year, while NBC surged by 17% but still finished a tick behind the third-place Alphabet.

For its part, “American Idol” remains a powerhouse, but it is seeing its greatest challenges since becoming TV’s No. 1 series nearly a decade ago.

In adults 18-49, it is being topped head-to-head in one of its regular half-hours (Thursday by CBS’ “The Big Bang Theory”), and in the weekly program rankings, its top night (Wednesday) is neck-and-neck with NBC’s “The Voice” on Monday. (Last week, “Idol” finished slightly ahead as the No. 1 show.)

Also, CBS’ Tuesday vet “NCIS” has been drawing more total viewers than “Idol.”

Overall last week, Fox’s 2.9 rating/8 share in adults 18-49 beat out CBS (2.5/7), with ABC (2.0/6) and NBC (1.9/5) further back. One year ago, Fox led this week by a whole ratings point over CBS (3.5 to 2.5) and ABC had a bigger edge on NBC (2.0 to 1.6).

Also last week, Fox edged CBS in 25-54 (3.5 to 3.4) while CBS cruised in total viewers (10.9 million to 8.7 million),

“American Idol” dominated its two hours Wednesday (6.1/16 in 18-49, 18.89 million viewers overall), on par with the previous two weeks. And though it dropped off on Thursday (5.1/14, 16.71m), it easily won three of its four half-hours.

On the scripted side, Fox failed to win any half-hours, coming closest Tuesday with “New Girl” (3.1/8, 6.47m); the 500th episode of “The Simpsons” ran third Sunday (2.6/7, 5.77m).

CBS saw good results for its strategy of rolling out its returning reality shows in the same week: “Survivor” was a solid second to “Idol” on Wednesday (3.1/9, 10.79m); “Undercover Boss” had a surprisingly strong start in its new Friday leadoff slot (2.4/8, 10.78m), bagging the best demo score for any Friday series on any net this season; and the 20th edition of “The Amazing Race” (2.8/7, 10.34m) was up 12% from last year.

Also, “NCIS” (3.9/11, 19.59m) and “The Big Bang Theory” (5.1/15, 15.65m) were the week’s No. 1 drama and comedy, respectively, in 18-49, 25-54 and total viewers.

ABC saw “Modern Family” (4.7/12, 11.23m) and “Grey’s Anatomy” (3.2/8, 8.31m) decline opposite atypical competish from “American Idol,” but a crossover storyline with “Grey’s” lead-out “Private Practice” produced above-average numbers for that Thursday 10 p.m. show (2.6/7, 7.08m).

On Sunday, “Once Upon a Time” led its hour in demos (3.1/8, 9.84m) but “Pan Am” had a soft landing to its season (1.2/3, 3.77m).

For NBC, Monday remained in its corner thanks to “The Voice” (6.0/15, 16.28m), down just 10% from its big bow.

“Smash” followed with declines in its second episode (2.8/7, 8.06m) and then fell further on Monday of this week (2.3/6, 6.47m). Still, it has emerged as the highest-indexing broadcast drama among upscale viewers.

And on Sunday, “Celebrity Apprentice” (2.6/6, 7.42m) opened a bit lower than last year but led its second hour in key demos.

CW had a strong Thursday with “The Vampire Diaries” (1.3/4 in 18-49, 3.08m) and “The Secret Circle” (0.8/2, 1.82m), with the latter up week to week. But the net continues to struggle overall, and its Wednesday unscripted series “Remodeled” drew just 460,000 viewers; and in the target demo of adults 18-34, its 0.2 rating ranked behind six broadcasters and numerous cable nets.

Univision was paced by Thursday’s “Premio Lo Nuestro” music awards (2.4/6 in 18-49, 5.64m), which lifted the net to a third-place tie with ABC for the night in 18-49. For the week, the net ran second to Fox in adults 18-34 (1.5/6).,

On the cable side, AMC’s “The Walking Dead” followed up its record preem perf with another great number despite declines (3.7/9, 6.89m) — standing as Sunday’s No. 1 series in 18-49 and second among all primetime dramas for the week in 18-49 (behind “NCIS”).

MTV’s “Jersey Shore” continues to gradually decline (2.9/8 in 18-49, 5.35m), dipping below the 3 demo mark for just the second time in its last 25 regular airings but still narrowly winning its Thursday 10 p.m. hour in 18-49. The new show behind it at 11 p.m., “I Just Want My Pants Back,” grew its audience to a best-yet 2.04 million.