Nobody at the parts counter can guess which engine you have

Saturday, 8:40 a.m., and the line at the parts counter is already five deep. The place smells like new tires and the cherry air fresheners hanging by the register. The man behind the counter has the calm of someone who has asked "two-wheel or four-wheel drive?" more times than any human should. I'm holding a wiper blade I bought last fall that never fit. Nobody else knows that yet.

Heard in the aisle

"It's a Silverado. Twenty-fourteen, maybe '15. Engine? The big one, I think."

The real lesson: Bring the VIN. It's on the driver's side of the dash where it meets the windshield, on the sticker in the driver's door jamb, and on your registration and insurance card. It tells the counter your exact engine, trim and build. "The big one" isn't a part number.

"Coolant's coolant. I grabbed the green one. It matched my lawn."

The real lesson: Coolant types aren't interchangeable, and color stopped being a reliable guide years ago. Match the spec in your owner's manual. And never open a radiator cap or a pressurized reservoir cap on a hot engine. The system is under pressure and can spray scalding coolant. Wait until it's cool to the touch.

"Ten-forty in everything. My uncle ran it in his tractor for thirty years."

The real lesson: Your uncle's tractor isn't your sedan. Use the viscosity printed in your owner's manual or on the oil fill cap, because plenty of modern engines are designed around thin oils like 0W-20. The right oil, changed on schedule, does more than anything on the additive shelf. That's half of what it really takes to get a car past 250,000 miles.

"Just give me the biggest battery that'll fit. Biggest is best, right?"

The real lesson: Match the group size so it sits in the tray with the terminals where your cables reach. Then meet or beat the cold-cranking amps your manual calls for. Bring the old battery back for the core charge. When you swap it, wear eye protection. Take the negative cable off first and put it back on last.

"Front pads only. The rotors are fine. They're round."

The real lesson: Rotors have a minimum thickness stamped on them, so measure instead of eyeballing. If the pedal pulses when you brake, have them checked. This next part is no joke. Never work under a car held up only by a jack. Use jack stands rated for the vehicle on flat, solid ground, and chock the wheels that stay down. Brakes are on the short list in the car servicing that can't wait, and the kind that can.

"The check engine light's been on since March. At this point it's dashboard decor."

The real lesson: Get the code read. Many parts counters will do it for free, and a basic code reader is a smart thing to keep in the glovebox. A code points you to a system, not a guaranteed bad part. If the light is flashing, that usually means an active misfire. Ease off and get it looked at soon, because a misfire can damage the catalytic converter.

"Red to red, black to black, and then you sort of hope."

The real lesson: Hope isn't a procedure. Connect the first red clamp to the dead battery's positive terminal and the second red to the good battery's positive. Put the first black on the good battery's negative. The last black clamp goes to bare, unpainted metal on the dead car's engine or frame, away from the battery. Batteries can give off hydrogen gas, and that last connection is where the spark happens.

"I filled the tires to the number on the side. Fifty-one PSI. The ride's a little... bouncy."

The real lesson: The number on the sidewall is the tire's maximum, not your target. Use the pressure on the sticker in the driver's door jamb, and check it cold, before you've driven anywhere.

"Two wipers, please. Same size. They've always been the same size."

The real lesson: That was me, last fall. My daughter is eleven. In the parking lot she told me the passenger blade looked shorter than the driver's. It was, by a couple of inches, and she hasn't let it go. Front blades are often two different lengths. Look up your car or measure both before you buy, and don't forget the rear one if you have it.

"I touched the glass on the new headlight bulb. That's fine, right?"

The real lesson: Halogen bulbs don't like skin oil. It can create hot spots that shorten the bulb's life. Handle it by the base or wear clean gloves, and wipe the glass with rubbing alcohol if you slip. Replace headlights in pairs so they match in brightness and color.

"I'll borrow the tool and bring it back. Eventually. Mostly in one piece."

The real lesson: Loaner tool programs run on a deposit and on the honor of the last guy who borrowed it. Return it wiped down, in its case, every socket and adapter where it belongs, and sooner than they expect. Leave it better than you found it, even if it came to you greasy.

"It only squeals when it's cold. Same as me in the morning."

The real lesson: A cold-morning squeal often points to a worn belt or a weak tensioner. Look for cracks, a shiny glazed surface or missing chunks. Before you take the serpentine belt off, photograph how it's routed. Some cars have a diagram on a sticker under the hood. Plenty don't, and you won't remember.

Before you go

• A photo of the door jamb sticker. It has the VIN, and usually the tire size and pressures too. Know your engine size and whether it's two- or four-wheel drive.
• The old part, in a bag. Setting it on the counter next to the new one ends most arguments before they start.
• Photos taken before you took anything apart. Belts, hoses, wiring, the order the bolts came out. Your future self isn't as sharp as you think.
• Measurements. Wiper lengths, the numbers off the tire sidewall, the old battery's group size.
• A little humility. The guy behind the counter has seen your exact car, your exact problem and, in my case, your exact wiper blade.