1. Flat tire replacement
Most flat tires are caused by screws, nails or other sharp metal objects penetrating the tire. When you get a flat, it's important to mount a spare and take the damaged tire in to a local tire shop. Be wary when the salesman tries to sell you a brand new tire or pair of tires. Most flats can be repaired with a simple patch, so always inquire about a repair before agreeing to an expensive replacement.
2. Radiator coolant replacement
When you take your car in for a routine oil change, sometimes the mechanic will recommend that you also replace your radiator coolant. Radiator coolant is simply water mixed with antifreeze, and it doesn't degrade in the same way as oil. Unless your coolant is contaminated or hasn't been replaced in 100,000 miles, don't fall for this common upsell.
3. "Preventative" oil replacement
Modern cars don't require very frequent oil changes, and modern lubricants last much longer than they used to. If you have a newer car that requires synthetic oil, you can go as long as 15,000 miles between oil changes. Before agreeing to come back for another change in 3,000 miles, check your owner's manual. Changing your oil prematurely is wasteful and unnecessary.
4. Unnecessary air filter replacement
Your car's air filter should only be replaced when it's clogged with dust and grime. Your mechanic should visually inspect the old filter and determine if your car is due for a replacement. Visual inspection on most cars only takes a minute, and shouldn't cost extra. If a mechanic tells you that it's time for a replacement, ask to see the old filter and inspect it for trapped debris. If the mechanic tells you to replace the filter before he's even examined it, chances are it's a typical unnecessary upsell.
5. Overpriced replacement parts
Most auto repair quotes will have a labor component and a parts component. Repair shops make money by performing the necessary labor, and also by marking up the parts. When your mechanic gives you a repair estimate, ask how much the labor costs and how much the parts cost. If the parts are a major cost component, you can always compare prices with your local auto parts store. Since few consumers ask about part costs or know what's reasonable, unscrupulous mechanics often profit from consumers' ignorance.