Lug Nut Ledger

What to Check When Buying a Used Car (A Mechanic's List)

By Lug Nut Ledger · · 7 min read

Written by an ASE-certified mechanic and licensed Pennsylvania state safety and emissions inspector.

When a friend asks me to look at a used car with them, I don't start by kicking tires. I start on the phone, because most bad cars can be ruled out before you drive anywhere. Then I work through the car in the same order every time, so nothing gets skipped because the seller is chatty or the paint is shiny. This is that order, and you don't need to be a mechanic to do most of it.

What to bring

  • A bright flashlight. A phone light is not enough for underneath.
  • Paper towels or a white rag, so you can see fluid colors.
  • A small magnet wrapped in a thin cloth, for checking body filler.
  • An OBD-II scan tool that shows codes and readiness monitors.
  • A tread depth gauge (or a coin) and a tire pressure gauge.
  • Gloves, a mat to kneel on, a friend to watch the lights and exhaust, and your license and insurance for the test drive.

1. Before you go

Get the full 17-character VIN first. A seller who won't share the VIN is a seller to skip. With the VIN you can:

  • Run a vehicle history report. Look for title brands (salvage, rebuilt, flood, lemon buyback), odometer problems, accidents, number of owners, where it was registered and long gaps in the records. A clean report does not mean a clean car: many accidents are never reported.
  • Check title and theft records. The federal NMVTIS system shows title brands from state DMVs (vehiclehistory.gov lists the approved report providers). NICB VINCheck at nicb.org/vincheck is free and shows theft and insurance total-loss records from participating insurers.
  • Look up open safety recalls at nhtsa.gov/recalls. Open safety recalls are repaired free at a franchised dealer for that brand.
  • Search the model's known problems. Search "[year] [model] [engine] common problems," read the owner forums, and write down the two or three big ones so you can check them in person.
  • Get a fair market value from KBB or Edmunds with the right trim, miles, options, condition and your ZIP code. Note the private-party and dealer numbers. You'll need this when you work out how much to offer.

Then ask the seller some questions on the phone. Is the title in your name and in hand? Any loan on it? Why are you selling? Any accidents, warning lights, leaks or noises? Service records? When were the brakes, tires, battery and (if it has one) the timing belt done? Has it failed a state inspection or emissions test? And the big one: can I take it to a shop for a pre-purchase inspection?

Last, ask them not to start the car before you get there. You want to see a cold start. Plan to look in daylight, in dry weather, on level ground.

2. Paperwork at the car

Before you open the hood, match the VIN on the dash plate (lower windshield, driver's side), the driver's door-jamb label, the title, the registration and the history report. They all have to match. The name on the title should match the seller's photo ID, and if there was a loan, the lien should be released in writing.

Look at the inspection stickers too. In Pennsylvania, check the month and year on the windshield safety inspection sticker and, in emissions counties, the emissions sticker. (Here's what a PA inspection sticker does and doesn't tell you.) If it's a dealer sale, dealers must post the FTC Buyers Guide in the window. It tells you whether the car is sold "As Is" or with a warranty. Take a photo of it.

3. Walk-around and body

Stand back about 10 feet. Does the car sit level? Is any panel a slightly different color or shine than the one next to it? Then get close and look at the gaps around the hood, fenders, doors and trunk. They should be even and match side to side. Uneven gaps, overspray on rubber seals and trim, and wrench marks on fender or hinge bolts all point to past collision repair.

If you're in salt country, spend real time on the rocker panels, door bottoms, wheel arches, quarter panels and the trunk floor under the mat. I go deeper on this in how to spot rust and frame damage.

4. Tires and brakes

Measure tread on the inner, center and outer grooves of every tire. In PA the minimum is 2/32", and nothing lower passes inspection. Other states set their own limits. Plan to replace tires well before that, and price it in. Uneven wear can point to alignment, suspension or inflation problems.

Read the tire age too. The last four digits of the DOT code are the week and year it was made, so "2321" is week 23 of 2021. Old tires can be unsafe even with good tread. Look through the wheels at the rotors and pads. In PA, brake pads need at least 2/32" of lining, and you often can't see the inner pad without pulling a wheel, so ask the shop to measure it at the pre-purchase inspection.

5. Under the hood, engine cold

Touch the hood and the upper radiator hose. If it's warm, ask why. A warm engine can hide hard starting, cold-start noises and smoke. Never open a hot radiator or coolant cap.

  • Oil: level between the marks. A milky, mayonnaise-like film on the dipstick or under the cap can mean coolant in the oil. A little residue under the cap can also come from short trips in cold weather, so weigh it with everything else.
  • Coolant: check the overflow tank only. Look for an oil film, rusty brown color or sludge.
  • Brake fluid: between MIN and MAX. Clear to amber is fresh; dark brown is overdue.
  • Leaks: wet oil or crusty residue around the valve cover, oil pan and filter, and dried coolant crust around hoses, the radiator or water pump.
  • Belts, hoses and battery: cracked or glazed belts, soft or swollen hoses, corroded terminals and the date on the battery label.

6. Cold start and smoke

Have your helper start it while you stand at the back, then the front. It should start promptly and settle to a steady idle. Listen for knocking, loud ticking, a belt squeal or a rattle in the first few seconds. Watch the exhaust: blue or gray smoke is burning oil; thick white smoke with a sweet smell that keeps going after warm-up is burning coolant; thin white vapor that clears as it warms up on a cool or damp day is normal condensation; black smoke means it's running rich. Any steady smoke after warm-up is worth a professional opinion.

7. Interior, warning lights and the scan tool

Key on, engine off: the check engine, ABS, airbag, oil and battery lights should all light briefly for their bulb check. A light that never comes on may have been disabled. With the engine running, they should all go out.

Plug in your OBD-II scanner (1996 and newer cars) and read stored and pending codes. Then look at the readiness monitors. If several say "not ready" or "incomplete," the codes were probably cleared recently or the battery was disconnected. Ask why. Then try every switch, the heat and A/C, and sniff for musty smells or damp carpet.

8. Underneath

With a flashlight and a mat, and never under a car held up only by a jack, look for fresh drips, kinked or freshly welded frame rails, rot at the subframe and suspension mounts, and heavily scaled or patched brake and fuel lines. Check that the catalytic converter is there and the exhaust hasn't been cut.

9. The test drive

Drive it for 20 minutes or more if you can, on a mix of roads, with the radio off. Shifting, brakes, steering, noises and the temperature gauge all get their own guide: the test drive checklist. When you get back, let it idle, look underneath again for fresh drips, and re-scan for pending codes.

Walk away, or negotiate?

Not every problem is a reason to pass. Worn tires, brake pads and rotors, an old battery, light oil seepage, dents, a missing second key or a state inspection that's due soon are things to price in and negotiate. These are the ones I'd walk away from (or at least get a professional opinion on first):

  • Title problems, or a VIN that doesn't match
  • A seller who refuses a pre-purchase inspection at a shop you choose
  • Structural rust, frame damage or poor structural repair
  • Flood signs
  • Milky oil plus sweet white exhaust smoke, or overheating
  • Heavy blow-by or steady blue smoke
  • A transmission that slips, flares or shifts harshly
  • An airbag light on, or signs a warning light was disabled
  • Codes recently cleared and monitors not ready, with no good explanation
  • Mileage that doesn't add up, or emissions parts removed

Then get a pre-purchase inspection

Everything above helps you decide whether a car is worth paying a shop to look at. It doesn't replace that. A shop with a lift can check the frame, lines and suspension properly and scan all the modules, not just the engine. Ask for a written list of findings with repair estimates. That list is your negotiating tool.