Cars these days are so complex—and most car owners are so unfamiliar with how they work—that dishonest or incompetent mechanics often can convince customers to agree to services their vehicles don’t need. Bottom Line ­Personal asked veteran mechanic and popular YouTube car-advice expert Scotty Kilmer what car owners should watch out for at the car-repair shop…

Oversold Services

Certain automotive services are recommended much more frequently than they’re truly needed…

Transmission flushes. Transmission fluid should be changed according to the vehicle’s service schedule found in the owner’s manual—but changing is different than flushing.

When an automotive fluid is changed, it’s drained and replaced. When it’s flushed, more aggressive methods are used to ensure that essentially all of the old fluid is removed before the new fluid goes in. Problem: Those aggressive methods sometimes can dislodge debris that could then settle in narrow parts of the transmission or even damage seals and valves, which can cause problems for you down the line. There’s a good chance the auto-repair shop is recommending a flush only because it costs more than a normal fluid change.

Also over-recommended: Other fluid flushes, such as coolant flushes.

Agree to a flush of any kind only if it is recommended in the manufacturer’s service schedule for your vehicle.

Tune-ups. There’s really no such thing as a tune-up with modern cars. In the old days, tune-ups included replacing points, cleaning and adjusting carburetors, setting ignition timing and changing plugs. But modern cars don’t need any of that. Potential exception: Spark plugs—but even they don’t need to be replaced often. Most vehicles now come with iridium plugs that last 100,000 miles or more.

If a mechanic says “tune-up,” ask, “What are you actually going to do?” Question why it’s needed if the answer doesn’t match the routine maintenance recommended by your car’s service schedule.

Fuel-injector cleanings. This isn’t something that modern cars tend to require as routine maintenance—and that’s why it’s not mentioned in most vehicles’ manufacturer service schedules. Possible exception: Your car might benefit from a fuel-injector cleaning if it has been stalling or idling rough.

Alignments. Your car might need an alignment if it shakes at highway speeds…pulls to one side…and/or the tires are wearing unevenly. Exception: If the vehicle is in an accident, a four-wheel alignment should be done by the shop handling the repairs. The insurance company is less likely to pay for a post-accident alignment if you don’t realize it’s needed until later.

Replacing shocks and struts. Before bringing the vehicle to a repair shop, take a quick look at your car’s shocks and struts. If these parts are covered in oil, they’re likely leaking and need to be replaced. But if they’re relatively dry, use your phone to take a photo of each one.

If the mechanic calls you over to look at these suspension parts when your car is up on the lift and points out that they’re coated in oil and must be replaced, show him your photos and ask how that oil appeared suddenly. Whatever he/she says, don’t believe it—the truth is that he sprayed oil on them to convince you to replace them, an old trick and an obvious sign that you need to take your car elsewhere.

Signs that shocks or struts legitimately need to be replaced: In addition to leaking oil, the car likely will lean through corners and have a bumpier ride than it used to and the steering wheel probably will shake when the car hits a bump at highway speeds.

Oil changes every 3,000 miles. The sticker that an oil-change shop puts on your windshield might advise you to return in 3,000 miles for an oil change. Reality: If you use synthetic oil, you can safely go 5,000 miles between oil changes and potentially farther depending on the vehicle and how it’s driven.

Warning: When you get an oil change, pay extra for a high-quality oil filter. Motor oils have improved over the years, but basic oil filters can’t be trusted to last 5,000 miles. The top-of-the-line filter made by well-known brands such as Bosch, Fram and Mobil 1 might cost several times as much as these brands’ entry-level filters—perhaps about $15 to $20 instead of $5 to $10—but they’re worth the price.

Replacing brake rotors. Brake shops almost always recommend that brake rotors be replaced each time the brake pads are replaced, significantly increasing the bill. Rotors really do need to be replaced more often than in the old days—today’s rotors are thinner and more prone to warping—but that doesn’t mean it’s always necessary.

Before taking your car in for brake work, choose a safe, open stretch of road and brake fairly aggressively from a starting speed of 45 to 55 miles per hour. If your front rotors are warped and require replacement, the steering wheel will vibrate during this braking test. If your rear rotors are warped, you will feel the brake pedal pulsing up and down under your foot. If you don’t feel these things, push back against a rotor-replacement recommendation or seek a second opinion.

Words to Watch Out For

Watch your wallet if you hear the following words from a mechanic…

“I wouldn’t drive it another mile.” This scare tactic is designed to convince you to have expensive repairs done immediately. You probably should drive away from a shop that says this…but since there is some small chance that the warning is warranted, the safer option is to have the car towed to a different shop for a second opinion.

“It’s a big job, but I can have it ready for you by 5:00.” A mechanic who stresses quick turnaround when recommending expensive work might have you pegged as someone who cares more about the convenience of getting your car back by the end of the day than about the bill. Seek a second opinion.

Free oil change/free inspections. The only reason repair shops ever advertise anything for free is because they want to get you in the door so they can discover—or invent—problems that require expensive repairs. Trustworthy shops don’t have to resort to freebies to attract customers.

Dealer service schedule. If you take your car to a dealership service department, you might be told that it’s overdue for maintenance according to the dealer service schedule. What the shop won’t mention is that this dealer service schedule is significantly more aggressive—and more costly for car owners—than the manufacturer service schedule in the car’s owners’ manual.

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