Used Mazda Dealer Said I'm Not Allowed To Use An OBDII Scanner During My Test Drive of a 2017 Mazda CX-3

In an earlier article, we learned about the red flags you should watch out for whenever buying a used vehicle. One is a check engine light warning on the dash or display during your pre-purchase inspection. But what if the seller refuses to let you plug in an OBDII scan tool like this used Mazda shopper experienced to look for engine or other system issues?

Is he being scammed?

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

Scan Tool Check Refusal

According to the OP, here is a summary of what happened:

"I just got back from test-driving a 2017 Mazda CX-3. Before the test drive, the salesman said, 'I have to ask you to please not use your OBDII scanner―it's aftermarket, and it will mess up the car computer system, so when we take it back to scan it ourselves, it will show codes that aren't actually codes'".

However, just two days earlier, at the same dealership but with a different salesperson, the OP took a test drive on a 2019 Mazda model using his scan tool, and the salesperson had no problem with it.

The OP questions whether the salesman who refused to let him use his scan tool on the 2017 model was trying to “pull a fast one on me." His reasoning is that if the salesperson was correct about scan tools causing damage, then it would be a policy with all Mazda dealerships.

Now here's the thing, I was at the same dealership two days ago test-driving a 2019 Mazda3. I had my scanner with me before, during, and after the test drive. I had a different salesman then, and all he said was, 'Oh, cool, you have an OBDII scanner.' That's it. He didn't say anything about how I'm not allowed to use one or that it's going to mess up the car. It's just a standard OBDII scanner off Amazon (the classic bright yellow one with 40k+ purchases)."

The OP elaborates further on his skepticism about whether a scan tool can damage a car.

I've used this scanner on my mom's Mazda and my Corolla for years, and we've never had any issues, and our mechanics have never said anything."

Can a Scan Tool Cause Damage to a Car?

The majority of the thread posts on the forum did little to answer the OP's questions satisfactorily.

Many claimed both "yes" and "no" that a cheap scan tool plugged into the OBDII port can and cannot affect a vehicle's operation.

There appeared to be confusion over whether the OP's scan tool was simply a one-way communication (unidirectional) device that only reads data from the engine's ECU. Or that it can communicate back (bidirectional) directly with the ECU, which could adversely affect the ECU's operation.

In either case, this distinction is not strictly relevant.

What is relevant is that anytime a digital system (like a Car's ECU) is electrically connected to another device, there is always the chance of corrupting the signal(s) between the two and causing the ECU to become "hung up" until a reset or other function is performed.

One good example is old-style gaming cartridges.

One of the best ways to cause havoc with your gaming station was to insert and remove a game cartridge without turning off the system first while the game was in play mode.

Furthermore, according to an informative instruction manual titled "Automotive Diagnostic Systems: Understanding OBD-I and OBD-II," the ECM is often software updated in today's cars without the owner's knowledge. This means that a scan tool must also be updated to remain fully compatible with the ECM.

While a not-up-to-date scan tool is unlikely in most cases to cause actual "damage" to the ECU, it could result in a malfunctioning ECU, necessitating a trip to a garage for proper resetting.

What the Scan Tool Industry Says About Scan Tool/ECU Problems

That said, however, a car's ECU can be damaged with a scan tool.

In an informative article from Ancel―a professional manufacturer of automotive products including OBD2 code readers, battery testers, jump starters, and other auto-related tools―titled "11 Ways OBD2 Scanner Could Mess Up Your ECU," there are 11 certain conditions that can lead to potential damage.

Related article: Best Car OBD2 Code Scanner and the Manual to Help You Understand How to Use It

Here is a summary of what the article states:

1. Poor Quality Scanners―Not all OBD2 scanners are created equal. Inexpensive, poor-quality scanners may contain substandard components that either emit incorrect voltage levels and can fry your EMC chips or cause data corruption.

2. Incorrect Usage―Misusing a scan tool during connecting or disconnecting it from your OBDII port can cause communication errors or short circuits. Many scanners require a specific start-up procedure with the ignition on and the engine off before plugging the scan tool into the OBDII port.

3. Software Conflicts―Using incompatible or outdated software with your OBD2 scanner can cause conflicts, leading to corrupted data or unintended changes to the ECU's settings.

4. Unstable Voltage―The ECU is sensitive to voltage fluctuations. Unstable voltage during diagnostics, often caused by a weak battery or alternator issues, can harm the ECU, especially if the scanner draws its power from a vehicle battery that has already been compromised.

5. Physical Damage to ECU Ports―Forcing connections or using a damaged scanner can physically damage the port, leading to poor connectivity or even short circuits, which can include the ECU.

6. Power Surges or Electrical Interference―OBD2 scanners that lack proper insulation or voltage regulation can inadvertently introduce electrical interference or amplify power surges that can fry the ECU chips.

7 and 8. Incompatible Protocols and/or firmware―A mismatch in communication protocols and software updates in a scan tool can cause erratic behavior in the ECU, including misinterpretation of data or unintended changes to the engine's settings.

9. Using the Scanner While the Vehicle is Running―Connecting or disconnecting the OBD2 scanner while the engine is running can result in a sudden change in electrical load, disrupting the ECU's operations and/or damaging it.

10. Multiple Devices Connected Simultaneously―In some cases, users might attempt to connect multiple diagnostic devices to the OBD2 port simultaneously. However, the ECU is designed to handle specific input and output patterns, and introducing multiple sources of communication can disrupt these patterns, leading to potential damage.

11. Inadequate Grounding―If the OBD2 scanner or the vehicle's electrical system has poor grounding, it can introduce electrical noise or create ground loops, interfering with the ECU's operation or even causing permanent damage to the ECU.

Was the Car Dealer Yanking the OP's Chain?

Whether or not the car dealer was attempting "to pull a fast one" on the OP is subject to debate that needs more info before reaching any conclusion.

It is unlikely that Mazda has a global "no-scan tool policy" regarding used car sales. However, dealerships can and do have their own personal quirks and policies.

As post contributor NemesisOfZod accurately points out, sound reasons why the car dealer did not want the OP to use his scan tool include:

Because your scanner could cause issues.

They're not insured for your mishaps.

So, unless you're willing to leave a deposit to cover the potential liability, then that's their dealership policy."

In other words, if it were YOUR car you were selling and someone wanted to plug in a cheap diagnostic tool of unknown origin by someone off the street with unknown user qualifications, wouldn't you be hesitant, too?

Tell Us What You Think: Have you ever had your car's ECU possibly damaged by a scan tool? Let us know about it in the comments section below.

For additional articles related to OBD scan tools, here are two for your consideration:

1. 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
2. Save Thousands Yearly in Scam Car Repairs and Maintenance With This Guide

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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