Tips for buying a used car for people that know nothing about cars

I’m a mechanic/auto shop owner. I figured I’d put some tips together for anyone interested in buying a used car but doesn’t know anything about cars. Obviously the best option is to take it to an auto shop to have it thoroughly inspected by a professional before you buy it. The keyword here is BEFORE you buy it. I don’t know how many people I’ve had come to my shop for an inspection after they bought it….. The $150 or so you spend is worth not buying something that may have thousands of dollars of problems.

Here are the 11 tips for buying a used car for people that no know nothing about cars

  1. Buy a cheap OBD scanner/reader
  2. Crank the engine without starting it
  3. Check the fluids
  4. Start the engine and listen for any noises
  5. Look for maintenance records
  6. Check that everything works on the car
  7. Look under the hood and look for any hokey work
  8. How does the car look?
  9. Check the tires
  10. Try and stay away from used car dealers
  11. Test drive the car

Buy a cheap OBD scanner/reader.

You don’t need to spend a lot, as they’ll all do what you need here. A Bluetooth OBD reader and the TORQ app for you phone seems to be pretty popular/good choice. Practice using it on some cars. Some of the stuff I'm going over may seem daunting, but if you practice it on a car a few times, you'll get the hang of it, and this will all make sense. You could be saving yourself thousands in repair here, so take a few hours to practice it. It's not hard once you do. You should be able to do all of the stuff I'm going to talk about with your scanner in under 5 mins total.

You don’t have to worry about reading and interpreting data. The main thing you want to do is check for codes in the engine and transmission ECUs. ECU stands for Electronic Control Unit. Basically, it's the computer that controls the engine or transmission. They are sometimes referred to as ECM, or Engine Control Module, and TCM, or Transmission Control Module. Sometimes they'll be referred to as PCM, or Powertrain Control Module. This is what it's called when only one computer controls both the engine and automatic transmission. The PCM may be one physical computer, but logically, it's 2 computers. So if you connect your scanner to a PCM, you'll still see two separate options, one for the engine, and one for transmission. Do note that if you have a manual transmission, there won't be a transmission computer.

So when you connect to each one, there should be no codes in either. If there are codes, there’s an issue. It may be minor, it may be major. Google it if you want, but not knowing what the codes mean, your best bet is to walk away. If your scanner is a better one, you can also check other modules (computers) for codes. However, it’s pretty common on newer cars, especially European, to find obscure codes in obscure modules. Normally they’re not an issue. Focus on the Engine and Transmission. ABS (antilock brakes) and SRS (safety restrain system…airbags, seat belts, etc) modules normally shouldn’t have codes lingering either. Make sure to also check after test driving. The codes may have been reset by the seller to hide a problem (more on that in the next paragraph). They may have returned during your test drive, so check again!

Use the scanner to check the monitors on the engine ECU/Computer. Monitors are a series of self-checks that the ECU does on the engine. All applicable monitors should be set (passed/complete). They get reset when you clear the check engine light, or when you disconnect the battery (usually). If all of the monitors haven’t passed, then it’s quite likely the person selling it has reset the check engine light recently (maybe trying to hide a problem), or there’s a problem that isn’t allowing the monitor to complete. Not a good sign. Walk away. To complete all of the monitors can take quite a few miles and sometimes several days. So there's a good window there for you to catch someone doing some hanky panky.

Bosch OBD diagnostic scanner tool
Bosch OBD diagnostic scanner tool

Crank the engine without starting it

What you want to do is listen to the engine during a continuous crank. On American cars and on Mazdas, this is easy, as they have what’s known as a Clear Flood Mode. You turn the key to the on position, wait a few seconds, depress the gas pedal all the way, then try to start it. The engine will crank away without starting for as long as you hold the key (or in the case of a push-button start, until you hit the button again). If the engine starts, quickly let off the gas so you don’t rev up the engine too high and try it again. You’ll want to listen to it for a good 10 seconds or so.

This is a very easy way to check compression on an engine. The main thing you hear when cranking an engine is the electric starter working to try and spin the engine. As a piston comes up and compresses the air, the starter has to work harder to spin the engine, and the speed/pitch of the starter changes. Once the piston comes back down, it’s easier to spin the engine, so the speed/pitch changes back, and then repeats as each consecutive piston moves up in the compression stroke. Every engine sounds different, but they all should have a very steady rhythmic starting noise. Kind of a WAAA WAAA WAAA WAAA WAAA. If one or more of the cylinders has low compression, you will hear the starter have an off-rhythmic sound that repeats.

So for instance, if you have a 4 cylinder engine with one low compression cylinder, it would sound like WAAA WAAA WA WAAA WAAA WAAA WA WAAA WAAA WAAA WA WAAA, etc. Every 4th pitch change will sound different than the other 3.

For reference, here’s what a normal cranking sound should be:

Here’s what a low compression cylinder cranking sounds like. It’s at about:55 secs:

If you listen closely, you’ll hear the unsteady rhythm. Loss of compression is usually piston rings, valves, or head gasket. All costly. If the engine sounds funny when cranking, walk away.

If the car you’re looking at doesn’t have a clear flood mode, you can remove the fuel pump relay in the engine compartment fuse box, which turns off the fuel pump. Most cars have one, but some don’t. Some cars can be a real PITA to disable it from starting. Google the car you’re looking at with something like “YYYY Make Model clear flood” or “YYYY Make Model disable fuel pump” and see if there’s an easy way to achieve this. I'm sure there will be a Youtube video showing you exactly how to do this.

Practice this beforehand on cars you have access to if possible to tune your ear to the sound. You can also do this periodically on your own car to see if any problems are arising.

Check the fluids

All of the fluids will have minimum/maximum marks. If fluids are low, someone has not been maintaining the car well, or it has developed a leak. Not a good sign. When checking the engine oil, it should range from clear to black. If it looks like a chocolate milkshake, there is a major problem. RUN AWAY. You can also smell the oil on the dipstick to see if it smells like gasoline. If it smells like raw gasoline, the engine is either injecting way too much fuel or you have bad piston rings. Either way, they're bad. If the oil level is WAY above the full mark, like an inch or more, then either some other fluid is making its way into the engine (very bad) or someone sucks at doing an oil change.

Remember to check the oil with the engine off and on level ground. Some new cars don’t have dipsticks (mainly European). If so, you’re SOL. Check the coolant in the reservoir. It should be green, pink, red, yellow, orange, blue, or purple, depending on the manufacturer. If it’s rusty, walk away. Remove the radiator cap (only if the engine is cold) and look at the cap and in the radiator. If you see any rust or chunky/gritty brown stuff, walk away. If it’s just water, walk away (be careful here, Ford’s yellow coolant almost looks clear). Check the automatic transmission fluid. For the most part, it should be read, but sometimes amber or green. It may be black. That’s dirty. Not a deal-breaker, but they haven’t been keeping up on maintenance. It should not smell burnt, though.

That’s bad. If it looks like strawberry milkshake, that’s really bad. Also, remember that you should check the level on automatic transmissions while the engine is running in Park and after driving it and getting the transmission good and hot. The only exception is most Hondas. That’s checked after driving but with the engine off. Google it for the car you plan on looking at to make sure. Many new cars don’t have a transmission dipstick, so again, you’re SOL there.

Checking the car fluid
Checking the car fluid

Start the engine and listen for any noises

The engine should be cold. If it’s at operating temperature, the seller may have warmed it up to hide some cold start engine noises. Be wary. If it makes any noises, walk away.

Look for maintenance records

If it has consistent oil change records at an oil change place, at least they’ve been changing the oil. Unfortunately, oil change places only check easy profitable stuff. It’s better than nothing, though. If the records are all at an independent shop, that’s better. Indy’s will usually do a pretty thorough check up on the car when servicing it. If it has all dealer records, that’s the holy grail. Dealers will find any nick nack that’s wrong and upsell it.

They also commonly don’t do thorough diagnostics (this is an unfortunate effect of the way dealer shops operate). So if it needed repairs, on top of having new parts that needed replacing, it may have other new parts that it didn’t even need. Plus those new parts will be good quality OEM parts, not chines junk of questionable quality.

Check that everything works on the car

Check the A/C, the heater, the windows, the locks, the mirrors, the head/parking/brake lights, etc. If the owner neglected to fix obvious problems, what else did they decide not to fix?

Look under the hood and look for any hokey work.

Random zip ties holding things on, tape, broken plastic pieces, a battery that can move around if you push on it, wires hanging, etc. If it looks like unprofessional work has been done on what you can see, how bad is what you can’t see?

How does the car look?

Is it dirty, full of scratches, stained? If the owner cares so little about the interior/exterior, they probably have the same attitude towards the mechanical part of it.

Check the tires

Aside from general conditions, do they all match? If all the tires are different, they’re cheap/broke

Proper tire inflation is very important
Proper tire inflation is very important

and have probably cheaped out on a lot more than just tires. Lay your hand flat on the tire tread and light feel around the tires. If you feel a repeating pattern of flat spots/dips, you have suspension problems.


Try and stay away from used car dealers.

Used car dealers get the majority of their cars from auctions. A lot of cars that go to auction are sent there by someone that doesn’t want it, usually because there are problems. Not all, but many. New car dealers send trade-ins that are too old or the wrong make to put on their lot, and some of those are decent. However, the small used car dealers usually buy the bottom of the barrel cars at auction.

They’ll fix the minimum needed with the cheapest parts possible to maximize profit. They’ll make it look pretty, though. Good chance you’re buying a polished turd. Not all used car dealers are bad, though. Check reviews. Look at what they have on the lot. If they have a lot of high resale value cars on the lot, they're buying the good stuff at auction. If all of their cars are under $10k, with a lot under $5k, move on.

Obviously, test drive the car.

Drive it at different speeds up to highway speeds. Brake easy, brake hard. Find a crappy road or railroad tracks to drive over. Make sure there are no noises or vibrations. Get it good and warm. When you’re done, open the hood and take a good whiff. Make sure there are no strong smells (like burning fluids or other things). Look under the car and see if anything is dripping or the bottom of the engine is covered in fluids (bring a flashlight, it can get dark under there).

Don’t be alarmed if you see water dripping under the car at about the same area as the base of the windshield/firewall. If the A/C or defroster was on, that just condensates from the A/C system. Touch it. If it’s not oily and looks/feels like water, it should be OK. On the other hand, If you’re test-driving a manual car, the clutch engagement point should be somewhere in the middle of the clutch pedal travel. If it’s right at the top or right at the bottom, clutch repairs are in the near future.

12. This one is a little more advanced, but not too difficult.

It’s also pretty important. You’ll need your OBD scanner. What you want to do is look at the engine data and search for the fuel trims. An engine computer injects fuel based on a bunch of sensor inputs. It has a base fuel map programmed into it that it references, based on those sensor inputs, and injects XXX amount of fuel. There is an oxygen sensor in the exhaust system that analyzes the exhaust gas and acts like a quality control inspector. It tells the computer whether it injected too much or too little fuel.

The computer then makes adjustments to that base fuel map to make sure it’s injecting the proper amount of fuel. Those adjustments are called fuel trims. A 5% fuel trim would mean the computer had to add 5% more fuel than the base map. A -5% fuel trim would mean that the computer had to reduce fuel by 5% from the base fuel map. In a perfect world, fuel trims would be zero. However, that’s rarely the case. Fuel quality, different atmospheric conditions, engine wear, engine or sensor problems, etc, make it so that the base fuel map is never perfect, so the computer is always adding or subtracting fuel (usually it’s adding, but sometimes it’s subtracting).

I don’t like to see a computer adding or subtracting more than 10% fuel. Any more than that and there may be a problem. Any more than 15-20% and there is definitely a problem.

So what you’re going to want to do is look at the data on the engine computer. You want to make sure you connect to the computer using the GENERIC OBD2 option on your scanner. Different car manufacturers will call these fuel trims by different names, and display the percentage in different ways. If you connect to the engine computer the standard way, you may be confused trying to find and read the fuel trims.

But if you connect using the generic OBD option, it’s always going to use a standardized display format across all vehicles. Some really cheap OBD scanners only connect using the generic OBD protocol. You’re going to see a long list of a bunch of different data. Scroll through until you find “short term fuel trim” and “long term fuel trim”. I'm not going to explain what the difference between those two data parameters are, as that doesn't matter here, and may end up being confusing. I'm just going to tell you what to do with the values you see.

Short term fuel trim, depending on your scanner, may be displayed as Short term fuel trim, STFT, ST, or ST%

Long term fuel trim may be displayed as Long Term Fuel Trim, LTFT, LT, or LT%

Let’s assume your scanner uses the more common STFT and LTFT designation. You’re going to see a number after the letters, so STFT1 and LTFT1. The number means the “bank” or side of the engine. A 4 cylinder engine only has one “side” so you’ll only see STFT1 and LTFT1. However, a V6 or V8 engine has two sides of the engine (3 or 4 cylinders on one side, and 3 or 4 cylinders on the other side, hence the V6 or V8).

The computer controls fuel independently for each side of the engine, so you’ll see an STFT1 and LTFT1 for one side of the engine, and STFT2 and LTFT2 for the other side. Don’t be alarmed if you’re looking at a V6 or V8 engine and you only see STFT1 and LTFT1. Many late 90s cars and some early 2000s cars didn’t control fuel separately for each side of the engine and lumped both sides into one bank.

When looking at the short term and long term fuel trims, you’ll notice the long term fuel trim number stays pretty steady, but the short term fuel trim number may change a lot. This is normal. What is important to note is that they are cumulative. So if STFT=4 and LTFT=3, then your total fuel trim is 7%. Let’s take a look at some examples on a V8:

STFT1 : 3 … STFT2 : 6

LTFT1 : 2 … LTFT2 : 1

So the total fuel trim on bank 1 is 5% (3+2) and the total fuel trim on bank 2 is 7% (6+1). Each bank is below +/- 10%. That’s pretty good.

STFT1 : -5 … STFT2 : 3

LTFT1 : 3 … LTFT2 : 1

Bank 1 fuel trim is -2% (-5 +3) and bank 2 is 4% (3+1). Again, that’s good.

STFT1 : 6 … STFT2 : 7

LTFT1 : 10 … LTFT2 :15

Bank 1 fuel trim is 16% (6+10) and bank 2 is 22% (7+15). That’s not good. Walk away from this one.

Here's one more that's a littlte different:

STFT1 : -20 … STFT2 : -20

LTFT1 : 22 … LTFT2 : 20

Hey, 2% and 0% total fuel trim on each bank. SWEET! this car is running almost perfectly! Well not really. Why is the LTFT adding 22% but then the STFT is taking a bunch of it back? There may be an intermittent issue going on here. So add the absolute values together as well (treat -20 as 20) and see what that total is. Here we have 42 and 40. There's some interpretation required here that you'd need some experience to do, but I'd say anything over 25 when adding absolutes is cause for concern.

Check these numbers with the engine running at idle, and rev up the engine and hold it at about 2500rpms and check it there. As I said, you may see the STFT number change pretty quickly, so just use the average of the numbers you see for that one. If you have someone with you, you can have them check the numbers while you drive as well.

Practice this on a car you have access to beforehand.

13. Last and not least, don’t trust the person selling the car. Trust your eyes, your ears, and your instinct. You don’t know this person, they may be lying about the car, or try and tell you that the thing you’re worried about is no big deal, it’s just this or that. Or they had a guy check it out and it’s a really easy/quick fix. Be patient and find the right car. If something is fishy or doesn’t seem right, move on to the next car. A car is a pretty big expense. Most people budget for the purchase price of a car and don’t consider there may be a considerable extra expense in fixing major problems. Minimize the possibility of those extra expenses by inspecting the car the best you can.

I would recommend running through these things, and any others you want to add, on your current car, your parents’ cars, friends’ cars, etc. Do it several times. Get comfortable in making these checks so that when you’re doing them in front of some stranger in their car, you won’t forget anything.