A lifetime warranty sounds like it would protect you against product failure for as long as you live. Right? But there's no single legal definition. So how do you know if a lifetime warranty isn't good for a lifetime?

When Don Roane's Craftsman ratchet broke, the Sears clerk made good on its lifetime warranty.

"He went and got me a replacement without any question," said Roane.

But that's not always the case. Nicole Van Scoten and her fiancé say when they bought their used car, the limited lifetime warranty was the deciding factor. Six months later the transmission blew, but their $7,000 claim was denied.

"We were really angry that, you know, they would not honour this warranty," Van Scoten said.

The warranty lists many things that are not covered, including "damage or failures resulting … from alteration." The original owner had made some changes to the car. But the couple says the sales team assured them those changes wouldn't affect the warranty -- something the dealership denies.

"You should never depend on spoken assurances. You have to read that fine print carefully. And don't assume that lifetime necessarily means your lifetime, it may simply mean the expected lifetime of the product," said Greg Daugherty of Consumer Reports.

And when a warranty says "limited lifetime warranty," it means limited. Consumer Reports found some patio furniture, for instance, which was covered by warranty only for the original purchaser. So the warranty is void if the furniture is resold.

As for paint, many carry a limited lifetime warranty, but only for the paint itself -- not the much bigger cost of repainting.

A popular treadmill's frame and motor are covered under a limited lifetime warranty, too. But if you need a repair, the warranty "does not include … freight charges," and that can get expensive.

The key is to read any lifetime warranty carefully. And get oral assurances in writing -- something Van Scoten and her fiancé learned the hard way.

Sometimes store staff is not up to date with policies, misinterpret them or simply forget. So before you make a claim, read the fine print and come armed with the facts. Be persistent and polite. Instead of demanding something, just nicely ask for help.

With a report from CTV British Columbia's Chris Olsen.