When your car breaks down or acts up, it is a major inconvenience — and can be a major expense. That is why finding a good repair shop is so important.

Unfortunately, nonprofit Twin Cities Consumers' Checkbook finds many repair shops that do lousy work, impose long delays, sell unnecessary repairs and give inaccurate estimates. But plenty almost always perform top-quality work quickly and for a fair price.

Checkbook's evaluations of 328 shops in the Twin Cities area include ratings for quality and price. Checkbook's ratings are based on more than 14,000 ratings it collected by surveying area consumers; a review of consumer-agency complaint records; more than 1,300 price checks by Checkbook's undercover shoppers; and other sources.

Until Feb. 7, Checkbook is offering free access to its ratings of local auto repair shops to Star Tribune readers via Checkbook.org/StarTribune/AutoRepair.

Fortunately, there are a lot of top-quality auto-repair shops in the area. Many shops were rated "superior" for overall quality by 90% or more of their surveyed customers. But there are also many shops you will want to steer clear of: Dozens of the businesses Checkbook evaluated got such favorable ratings from fewer than 60% of their surveyed customers.

Checkbook's ratings of area shops include a separate rating for price, derived from price quotes collected by its undercover shoppers for several carefully constructed repair jobs. You want to be sure a shop charges fair prices before you bring in your car because, like with most repair work, it is difficult to shop for price before you know exactly what needs to be done.

Checkbook's undercover shoppers found dramatic shop-to-shop price differences. For example, to replace the water pump for a 2014 Ford Escape, they found prices ranging from $270 to $649 among area shops. Hourly labor rates range from $80 to $190.

If you know what repairs you need, you can compare prices from shop to shop on your own by calling a handful for quotes. Shops might be able to tell you over the phone what is likely to be wrong and quote a price. If so, get quotes from several shops.

When shops can't determine what is wrong with your car based on your description, you will have to take it in for a diagnosis and estimate. Then, with estimate in hand — and assuming that the diagnosis is correct — check with other shops to see if the shop's price is fair. You don't have to pay more for good service: Checkbook found no relationship between the prices shops charge and the quality of their work.

Many consumers believe dealers offer better repair service due to access to proprietary knowledge, sophisticated diagnostic software and high-tech tools, not available at independent garages. That is not true. In fact, Checkbook found the opposite: On average, shops operated by nondealers were far more likely to satisfy their customers than dealerships — and offered lower prices. The nondealers were rated "superior" overall by an average of 85% of their surveyed customers vs. 69% for dealers. Prices at nondealers averaged about 8% lower.

Both dealers and nondealers subscribe to the same databases that provide repair instructions, diagrams and news from manufacturers. Although many car dealerships feature nifty-looking workstations, independents have the same tools and equipment

Checkbook's advice: If the work you need is not covered by a new-car warranty, use an independent shop.

With any shop, communication is critical. Checkbook advises:

• Give the shop a detailed written description of your car's symptoms. But distinguish between what you know and what you think you know. Don't guess.

• If possible, speak with the repair technician who will be working on your car. Service write-up personnel at large shops often know very little about car repair.

• Either get a written estimate in advance, or write on the repair ticket that no work is to be done without your approval based on a written estimate.

• Get a written, dated invoice that details charges for parts and labor, and the vehicle's odometer reading.

• Pay by credit card — you can dispute the charges if things go wrong and the shop isn't responsive.

• If the car is still not right when you get it back, immediately inform the shop in writing.

Twin Cities Consumers' Checkbook magazine and Checkbook.org is a nonprofit organization with a mission to help consumers get the best service and lowest prices. It is supported by consumers and takes no money from the service providers it evaluates.