When you take your car in for repairs, you're at the mercy of the garage.

The Automobile Protection Association and CTV's W5 did an undercover investigation to see if garages could properly diagnose and repair a simple problem.

The APA simply loosened the battery cable on two cars that were in otherwise perfect condition and took them to 19 randomly selected repair shops.

The secret shopper told the garages that sometimes the car just won't start.

Only six of the garages passed the test by finding and tightening that loose cable.

Four couldn't find the problem, but didn't do any unnecessary work.

Of the remaining nine, three recommended unnecessary work and six actually performed and charged for unnecessary repairs.

Canadian Tire at 3516 8th Ave. NE charged $507 to replace a perfectly good battery and two perfectly good spark plugs.

When the secret shopper asked the garage to return her original plugs, the shop gave her plugs from a different car.

"APA's spark plugs were about 1000 km old. The ones they gave back were spark plugs that were about 80,000 km old," says the APA's George Inny.

The Canadian Tire in Country Hills had the second worst result.

It charged $407 to replace a good battery.

The mechanic initially indicated the battery was "satisfactory" on the inspection form, but someone later crossed that out and marked "unsatisfactory".

"So we have the sense there that it's not the tech working on your car that makes the final decision. Someone in the shop might decide "hey you know, we gotta put some parts on" for one reason or another," says Inny.

For comment on these two failing results, we were referred to a vice president at Canadian Tire.

She provided an emailed response. In it she states that Canadian Tire is "...committed to operating with integrity... At no time was the mystery shopper told that recommended work was required when it wasn't...Canadian Tire is fully accountable to our customers and in any case where a customer has questions about the work done on their vehicle, we will be transparent and responsive."

Kal Tire on Edmonton Trail found and tightened the loose battery cable.

Problem is, it also replaced the ignition switch for a total of $355.

When contacted, Kal Tire maintained that the ignition switch core was loose, and because battery problems typically don't cause the "intermittent no start" trouble described it was reasonable to replace the switch.

The Kal Tire on 42nd Avenue also got it wrong, replacing the car's battery for $171.

Kal tire stands by that as well, stating that because the battery was unlabeled, its tech had to guess at the baseline number for a passing grade on the battery test and the battery failed based on the numbers used.

The Fountain Tire on 40th Avenue also failed, needlessly replacing the car's battery.

The bill totalled $233.

The shop's owner, Rick Ball took full responsibility and offered a refund.

"The buck stops here. I'm the guy that's responsible and we did it wrong."

Ball said it was either human or equipment error and replaced his battery testing equipment to prevent future mistakes.

The owner of Calgary's Midas shops also took responsibility for the failure at 16th Ave. NW. That shop unnecessarily replaced the car's battery for $187, based on erroneous battery test results.

In a written statement, he said "the technician relied on the electronic testing tool for the battery test results and missed the loose battery terminal connection. The battery terminal should have been checked...we have since then replaced the testing tool."

He pointed out that unlike some garages Midas does not pay its employees commission on parts sold to remove any incentive to oversell.

Shops that failed the APA test:

Midas: 624 16th Ave NW

Canadian Tire: 3516 8th Ave. NE

Canadian Tire: 388 Country Hills Blvd. NE

Kal Tire 3712 Edmonton Trail NE

Kal Tire 404 42nd Ave SE

Fountain Tire: 425 40th Ave NE