I Just Bought a Used Car. After About 50 Miles, The Check Emissions Light Came On. I Checked It Out At a Car Lot That Had a Scanner That Can Tell If Was Reset. And Sure Enough It Was Reset Shortly Before I Bought It

One of the most oft-repeated bits of used car shopping advice given anywhere is that before buying any used car, whether it is from a dealership, a relative, some guy curb stoning a great deal, etc., is that you should always seek the help of a mechanic to do that all-important prepurchase inspection.

This is an important issue. However, it is not uncommon to discover that whoever is selling the car will tell you that you cannot take their car to a mechanic for a prepurchase inspection. There is always that "liability" reason and why the only testing you can do with the vehicle is a short test drive around the block with the seller riding shotgun.

Unfortunately, that test drive is nearly useless when it comes to used cars.

Why Test Driving a Used Car is Nearly Useless

Why? Because a test drive will only reveal obvious problems with the car. This is why in a lot of advertisements with problematic cars, you will read a brief "Starts and runs good" description.

Sure, The test drive does help you evaluate for:

1. Performance: Acceleration, braking, and engine smoothness.
2. Handling: How the car responds to steering, cornering, and rough roads.
3. Comfort: Seat support, cabin noise, and overall driving ergonomics.
4. Functionality: Testing controls, displays, and features like air conditioning, radio, and lights.

But this is just one test out of several you should be doing that includes ensuring the Vehicle Identification Number (VIN) matches the car, the repair/service records, and the ownership; investigating a vehicle's history through services like CARFAX; comparing its actual value to models of similar age and mileage; and trusting your instincts―if something doesn't feel or sound right, this is a red flag.

Related article: Red Flag Used Car Dealers Do Not Want Buyers to Know About

Another Reason Why Road Tests Do Not Mean That Much

Even by following the tests mentioned above, you are still at risk of buying an expensive lemon because many car dealers and private sellers resort to a simple trick even a non-mechanic can do.

In fact, a relatively recent Reddit post from a r/UsedCars social media thread from the original poster revealed that he had fallen for one of the more common and least detectable scams many used car sellers resort to: clearing engine codes to ensure that the check engine light does not come on during a test drive.

According to the host of the Royalty Auto Service YouTube channel this week, this is a problem many used car shoppers fall for because they are unaware that some check engine light codes erased by an unscrupulous seller will not reappear until weeks or even months after the car is sold.

The good news, however, is that you can easily use a similar tool the scam artists use to verify whether that car has a history of check engine light codes you need to be aware of―before even bothering to take a car for a test drive.

Follow along with the host as he explains how this code-clearing scam works and the tool you can essentially plug and play to find potential hidden problems with a used car. If you prefer, a summary of the key points from the video is provided immediately below the video.

Everyone Should Always Do This Before Buying A Used Vehicle!

Video Key Points

Here is the least you need to know about how to protect yourself from a code-clearing scam with that used car you are considering buying:

1. Many used car dealers will not allow a car to be taken to your mechanic for an inspection. However, if they will allow it―do it!
2. A test drive will not reveal a hidden check engine light problem if the codes have been erased without fixing the problem. The check engine light will return, but it may be days, weeks, or even months later when it does.
3. A simple code reader will not reveal that the codes have been erased. Many will only tell you whether a code is detected.
4. A GEARWRENCH dongle, which you can buy on Amazon, is the tool you need to plug into a used car's OBD port and analyze with your smartphone to determine if any codes have been erased.
5. Catalytic converter and Evaporative system trouble codes are typically erased by scammers because they are common problems in older cars, are too expensive to fix, and are, therefore, tempting to pass on to the next car owner.
6. The check engine light is really a "warranty" light designed by the manufacturers and is not the most accurate indication of an engine or system problem.
7. The abbreviated "Incmpl" for "incomplete" is the message from the dongle you want to find indicating whether a code has been erased.
8. If the seller tells you that the used car has a new oxygen sensor installed, it is a red flag that there are problems, and the seller may have erased the indicative code.
9. Depending on your state, if an emissions test is due after you bought the used car, the testing station will likely discover the erased code and will want you to fix the car before it can be road-legal again, leaving you with an expensive repair bill. If you had tested for erased codes before buying, you would have saved hundreds to thousands of dollars.

One Final Important Point: If you find a code has been erased in a used car you are considering buying, take the info to the seller and ask whether the problem has been fixed; It is possible the seller did take care of the repair earlier and that you are not being scammed.

For protection, however, ask for a notarized statement or other agreement/guarantee against a potential repair in case the seller is not being totally honest with you. If they balk, it is time to pass on the used car and look elsewhere.

For additional articles related to the usefulness of scan tools, check out these two titled "Best Car OBD2 Code Scanner and the Manual to Help You Understand How to Use It" and "New Odometer Fraud Tool Scam Warning."

Timothy Boyer is an automotive reporter based in Cincinnati who currently researches and works on restoring older vehicles with engine modifications for improved performance. He also reports on modern cars (including EVs) with a focus on DIY mechanics, buying and using tools, and other related topical automotive repair news. Follow Tim on Twitter at @TimBoyerWrites as well as on Facebook and his automotive blog "Zen and the Art of DIY Car Repair" for useful daily news and topics related to new and used cars and trucks.

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