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How To Evaluate and Test Drive a Used Car


If you're going to buy a used car, then you want to get the best car you can for the money. Unless you're buying the car from someone you know, in most cases the inspection and test drive will be your only chance to evaluate the car that you're considering. Here are some ideas to help weed out the lemons.

Unless you're a gearhead, you may want to consider having a mechanic inspect the car you're considering buying. There are many mechanics who make a living from inspecting used cars for potential buyers. Hiring one of these experts is something you should consider if you're not mechanically inclined (or even if you are, but cars aren't your specialty). Most garage mechanics will also be happy to do this for you if they're not too busy with actual repairs, but usually they'll want you to bring the car to them. If the seller's okay with that, then it's a perfectly acceptable way to go.

Either way, be realistic in your expectations. Cars are very complex machines, and not even a skilled mechanic can guarantee that a car that seems fine today won't suffer an expensive-to-repair breakdown tomorrow. There are just too many parts, and many of them are not accessible for inspection. But a good mechanic can at least get an idea if the car seems well-maintained or if the seller is pulling any shenanigans to try try to cover up an obvious problem.

If money is tight, you may want to have a gearhead friend who's skilled in car mechanics accompany you to check out the car. If you live near a college or trade school that teaches auto mechanics, you may also be able to hire one of the students to help you out for a lot less than a professional would charge. A lot of them are pretty good.


Performing Your Own Used Car Inspection

If you're going to do your own inspection, here are some things you should check for. This is not an exhaustive list, by the way. I've selected the things that tend to be most revealing of a car's condition, but which an average non-mechanic should be able to do.

When showing up to inspect a used car, try to schedule the inspection for a sunny day. Be sure to bring a flashlight and an inspection mirror with you, as well as a pair of safety glasses to keep rust and dirt out of your eyes while you're looking under the car. Wear old clothes or coveralls because you're going to get a little dirty. A simple voltmeter or multimeter can also come in handy for checking the battery and electrical system.

If you have or can borrow an OBD2 code reader or scanner and know how to use it, that's a big plus. OBD2 codes can reveal a lot about the condition of a car's engine, emissions system, and transmission.

There are two kinds of devices that can read OBD2 codes: readers and scanners. Readers only show you what the codes are. Scanners can monitor many aspects of the car's systems in real time and are more expensive than readers. If you are or would like to be a shade tree mechanic, then a scanner is a better bet because it will help you to diagnose and fix problems. If you just want to read codes, then a reader is less expensive. There also are inexpensive phone and tablet apps that can be used as scanners if you purchase a compatible OBD2 Bluetooth adaptor.

Here's my suggested routine for inspecting a used car. Important Safety Note: Car engines have a lot of moving parts, and some of them only move intermittently. Keep your hands well away from the engine, belts, pulleys, fans, and anything else that looks like it may move.


How to Use an OBD2 Reader to Check a Used Car You're Thinking about Buying

In previous versions of this site, I mentioned taking OBD2 readings as part of the inspection, but some readers told me my explanation was inadequate and confusing. So let me go into a bit more detail.

OBD2 (or OBD-II) for "On Board Diagnostics Version 2" and is a diagnostic system built into all cars built since 1996, primarily for emissions-control reasons. It replaced an older, rudimentary system called OBD1, or simply OBD. The OBD2 system constantly monitors many aspects of the car's operation, and when some value is outside the boundaries that the system expects, it issues a "code" and lights the CEL (Check Engine Light).

An OBD2 reader reads OBD2 codes, and nothing more. An OBD2 scanner also reads codes, plus is also able to read a great deal of information about the car's systems in real time, as well as control some of the systems. Unless you know how to interpret this information, you can just use a reader. They're less expensive than scanners.

An OBD2 port on a car

The OBD2 port is usually located under the dash on the driver's side

To use an OBD2 reader, simply insert it in the OBD2 port, which almost always is located under the dashboard on the the driver's side. Turn the ignition key to the "ON" position without starting the car. Then wait for the reader to query the OBD2 system and report the response, which can take anywhere from a few seconds to a minute or more. The response will either be no codes, pending codes, or "Not Ready."

An OBD2 code doesn't necessarily mean that the part the code refers to has failed (although that's sometimes the case). It merely means that a value was outside the normal range. In other words, if the code refers to one of the many sensors in the car, it doesn't necessarily mean that the sensor has failed. The sensor may have failed, but it could also be accurately reporting a problem in whatever system it monitors.

The reason I mention this is because if a car has a code and the seller tries to tell you something like, "Oh, don't worry about that, it's just a ten-dollar part," that means nothing. The sensor might be perfectly fine and accurately reporting some other problem in the car which may not be such an easy and cheap fix. A good rule of thumb is to never buy a car with a code or a lit CEL without having a mechanic look at it first. If the seller won't let you have a mechanic inspect the car, then walk away.

Another time you should walk away is if you try to check for codes and the reader or scanner gives you a "Not Ready" message. What that means is that the seller either cleared the codes with a reader or scanner, or disconnected the battery, which has the effect of clearing the codes on most cars. It also has the effect of temporarily turning off the CEL for codes associated with tests that take several drive cycles to complete. Until the car has been driven enough to complete those tests, the CEL will remain unlit.

In other words, if the system reports "not ready," then the seller is probably trying to rip you off. They most likely cleared the codes so the CEL would not be lit when you test-drove the car. If you buy the car, the CEL will come back on once you've driven it a few times; and the seller's problem will have become your problem. The only other explanations for a "not ready" reading are that the seller just had some work done on the car that required disconnecting the battery, or that the car was sitting unused for a very long time and the battery went completely dead. In any case, I suggest you either walk away or consult with a mechanic before buying any car with a "not ready" OBD2 indication.

Another type of code that's cause for concern is called a "pending" code, which can be thought of as a code that the OBD2 system isn't yet sure that it wants to issue. Some codes require that a certain value be out-of-range more than a certain number of times before a code is issued or the CEL is lit. Until that happens, the code is stored as pending, and the light stays out while the system makes up its mind. Some pending codes can be signs that a part is getting ready to fail; so again, it's a good idea to talk to a mechanic before buying a car with a pending code.

If there are active codes, there should also be a lit CEL. If there are codes but no CEL, then it's possible that the seller pulled the bulb behind the indicator and is trying to pull a fast one on you. It's also possible (though very unlikely) that it simply burned out. Either way, it's a suspicious thing. Most codes also light the CEL. If that's not the case, then I suggest you walk away or consult a mechanic.

If there are codes and the CEL does light up, then write down the codes so you can talk to a mechanic about them. The same goes for pending codes. Some are easy and cheap to fix. Others, not so much.

Finally, just because the system is ready and there are no codes, that doesn't mean the car is in good shape. All it means is that the systems the OBD2 system is programmed to monitor are not reporting any out-of-range values. The car may still have problems that don't cause codes. So in other words, the absence of codes is a good sign, but not a guarantee that the car is problem-free.



How to Test-Drive a Used Car

Before test-driving a car, make sure that it has a current registration, inspection (if required), and insurance. Ask the seller, and if you have doubts, ask to see proof. You can be ticketed for driving a vehicle that doesn't meet these requirements.

My suggested routine for test driving a used car is as follows.

Yes, this is a pretty complex sequence. Its purpose is to test for problems that the car is likely to have by putting it through a compressed simulation of all of the common driving situations. Some of the tests are subjective: You're basically judging the car by the way it "feels." Others are designed to encourage the OBD2 system into issuing a code and lighting the CEL if there's a problem.

Even if you're not especially mechanically inclined, following this sequence will give you a better idea about the car's condition than just randomly driving it around the block. But in the end, it's still a gamble. If possible, you really should have a mechanic or a friend who's skilled in these sorts of things go with you, if you're in doubt.

The most important thing to remember is that there will never be a shortage of used cars. If something doesn't feel right to you, then consider walking away or lowering your offer accordingly. Used cars are placed on the market every day. It's much better to wait for the right one than to get stuck with a lemon.

I hope these suggestions have been helpful to you. Good luck in your car search!


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