LOS ANGELES — Sometimes a TV series needs a tune-up. Tweak the plots, juice the promotion and it may end up a challenger in the ratings race.Sophomore CBS sitcom “How I Met Your Mother” is nudging respectable but not stellar Nielsen numbers upward with such changes, keeping its central courtship saga alive for loyalists but mixing in more stand-alone laughs for newcomers.

An episode in which an embarrassing music video surfaces for character Robin (Cobie Smulders) made an Internet splash and is credited with a 10 percent-plus jump in the following week’s audience, to 9.8 million, said executive producers Craig Thomas and Greg Malins.

“We’re not `24,’ where people have that feeling of, ‘Oh, my God, I can’t miss tonight,”’ Malins said. “It’s comedy and people are going to watch when they watch. ... You try to make each episode enjoyable on its own.”

At NBC, “Studio 60 on the Sunset Strip” recently shifted from social issues and corporate clashes to romance, stepping up Matt and Harriet’s (Matthew Perry, Sarah Paulson) breakup angst and Danny’s (Bradley Whitford) passionate pursuit of Jordan (Amanda Peet).

But that failed to halt the ratings slide for the backstage Hollywood drama, which debuted last fall with 13.4 million viewers. Monday’s audience of 6.4 million was among the smallest yet for creator Aaron Sorkin’s costly post-”The West Wing” effort.

Sometimes, it turns out, minor repairs aren’t enough — a series needs a whole new engine.

It’s true for “Studio 60” as well as NBC’s “Friday Night Lights” and ABC’s “Lost,” shows that either failed to gain traction or have seen their audiences shrink. All need to act fast if they want a shot at survival.

There is some historical, if scant, support for hope.

Durable Western drama “Gunsmoke,” after losing its hit status in the early ‘60s, expanded to an hour, went from black-and-white to colour and moved to an earlier, more child-friendly time, said Tim Brooks, co-author of “The Complete Directory to Prime Time Network and Cable TV Shows.”

“As a result of all that retooling, it became a hit all over again with a whole new generation,” Brooks said.

But he cites other, less promising examples, including the 2002 sitcom “Good Morning Miami,” which underwent a fruitless makeover from slapstick to romance in its short life.

“Changes, especially subtle ones, aren’t even noticed by the audience which has decided what they think the show is,” Brooks said. “With so many choices and new programmes coming on, why go back to something previously rejected?”

Exactly, says one industry expert. You’ve got to give a failing show an element that’s new, that’s hot — along the lines of Heather Locklear, said veteran network executive Tom Nunan, now a TV and film producer (“Crash,” “The Illusionist”).

Nunan recalls being at Fox when “Melrose Place” debuted to audience indifference.

“We had a soap without a villain in it,” he said. “They were all nice people, really good-looking, in an attractive environment, but there weren’t the kind of spiky moments between bigger-than-life characters we had gotten to know and love on other big soaps like ‘Dallas’ and ‘Dynasty.”’

Enter Locklear as gorgeous, ruthless Amanda, an addition which “completely turned that show around,” Nunan said.

He’d like to see similar moves on “Studio 60” and the small-town football saga “Friday Night Lights,” both of which he ranks among TV’s best dramas but which he warns will fail if they merely try a “subtle course correction.”

At a January news conference, NBC Entertainment President Kevin Reilly expressed support for the series but withheld word on their fates. He said the time slot given “Friday Night Lights,” on Friday, will be revisited.

For one-time hit “Lost,” which dropped to its lowest rating ever this month, there’s no shortage of advice on the web or from the industry. Nunan, a self-described fan, advocates a two-pronged approach.

ABC must find a stronger lead-in for the show, which recently has aired after either its own reruns or sitcoms, and the producers need to focus the story lines on key characters: “Get those people in danger, have them sleep together, create those jealousies,” Nunan said.

In other words, sex sells.

But “Lost” could be irreparably damaged. That’s how it looks to Marc Berman, TV analyst for Media Week Online, although he is willing to suggest a battle plan for the show, including moving from its 10 p.m. EST Wednesday slot back to 8 p.m.