Nope, CarMD Won’t Void Your Warranty

By: Mike Allen, The Saturday Mechanic

Q: I bought a CarMD device a year or so ago. I’ve used it on a couple of my cars, and last week my CHECK ENGINE light came on. I hooked up the CarMD, and got the trouble code off the computer in the car. CarMD’s website was very helpful. I took the printed-out diagnosis with me to the dealership for the repair. When the service writer saw the printout, he immediately informed me that any repairs to my car from that point on would no longer be covered by the warranty, as I had used an “unauthorized device,” and had potentially damaged my car’s computer system. I still have almost two years of the warranty in effect—what should I do?

A: Your service writer is either misinformed, or is looking for a way to prevent you from making a warranty claim you’re entitled to. A CarMD device cannot damage your car’s computer, period. Like most retail code readers and scan tools, CarMD is a read-only device. These devices capture data, either in the form of trouble codes stored in the computer’s memory, or by accessing the stream of data moving around between the computer, the sensors that tell the computer what’s going on, and assorted devices the computer is controlling, like the fuel injectors and the ignition coils. A scan tool can send data back into the car’s data bus, in order to shut off the CHECK ENGINE light; but CarMD doesn’t do this. Some pro-grade (read: extremely expensive) scan tools can send data up to test some devices connected to the computer system. This is so the service technician can test them. Again, there is no way the scan tool can damage the computer itself.

There are aftermarket devices that will rewrite some parameters on the computer’s programmable internal memory. Let me start from the top: default values for things like fuel injection delivery rates and ignition timing are preprogrammed into the computer’s EPROM (Erasable Programmable Read Only Memory) chip at the factory. These values have a profound effect on emissions, economy and performance. Car manufacturers sometimes have to revise those values to correct issues that come along after the car is on the market, or to meet EPA emissions requirements, so it’s possible to use a factory scan tool or a dedicated EPROM programmer to change them. Also, using aftermarket equipment, like larger air intakes or low-restriction exhaust systems in the quest for performance may require a reflash of this memory. If this rewriting is done using anything except a factory-approved set of data, your warranty is indeed void.

However, there is no way that a CarMD product can alter the EPROM. Also, the dealer can access the EPROM data and verify which version of the data is installed. This will show the correct version, and he’ll have to back down.

Voiding your warranty for using a CarMD or other consumer-grade scan tool without rewriting the EPROM is prohibited by the Magnuson-Moss act, which specifically permits you to use or install aftermarket devices or parts that don’t affect the emission system. My suggestion? Go over this service writer’s head, to the manager of the service department, or the dealer principal. If that doesn’t help, ask for an appointment with the car manufacturer’s district service rep the next day he’s in town. These reps typically are responsible for a number of dealerships, and are only around once or twice a month.

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Mike Allen is a longtime automotive expert and journalist.  He owns and publishes, a website dedicated to people who still believe in repairing their own cars.  Mike spent 25 years at Popular Mechanics magazine, finishing his stint there as Senior Automotive Editor.  His Car Clinic column was syndicated by the NY Times, reaching nearly 20 million readers weekly.  Mike has appeared on national TV numerous times, including Monster Garage, Mythbusters, NBC Dateline, Paula Zahn and numerous regional shows.  He’s also a current ASE-certified Master Technician. 

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  • By Brittany Halstead, May 15, 2012 @ 9:43 am

    Good to know! Never thought about warranty issues. Always just assumed the only problem would happen if you erase the codes, not just read them. Of course that would only be a problem because then the service tech would not be able to see what it originally said! Luckily the CarMD was intuitively designed not to interfere! I love mine. Thank you.

  • By Car Repair Guy, May 23, 2012 @ 11:09 am

    Many people do not realize that vehicles have responsibilities and how important it is to maintain them. Most of the time, we take our cars in for a checkup or for repairs when a problem actually occurs. With proper maintenance, you will prevent major problems from happening. One problem can lead to another, which is why it is important to solve minor issues on a regular basis.

  • By Pete C., July 5, 2012 @ 10:54 am

    I bought a 2010 vehicle no problem yet but was wondering if i use car MD to view my cars data now will the car dealer know that car MD was used and when?

  • By Ivy from CarMD, July 6, 2012 @ 1:11 pm

    Hi Pete,
    Don’t worry! The car dealer will not be able to see if you’ve ever used the CarMD device to diagnose your vehicle. Thank you!

  • By Nunya, August 17, 2012 @ 4:17 pm

    If it can’t damage the cars computer, then how do you explain the recurring problem of scrambled computers in some vehicles – specifically the Toyota 4 Runner.
    And before you deny any knowledge of this, make sure you check with the Corporate Offices who were made aware on or before 08/24/2011.
    Also, anything plugged into a computer can potentially damage it. Especially if it has its own power source.
    I suggest searching various consumer reporting sites, the Better Business Bureau n sites like “Ripoff Reports”. Also u can check dealer websites for complaints, reviews n feedback such as the on

  • By Ivy from CarMD, August 21, 2012 @ 11:34 am

    Hi Nunya,

    I work in the Technical Service Department here at CarMD and was asked to review your comments regarding the possibility of Toyota4Runners and other vehicles having their PCM (computer) potentially damaged by various devices. I will assume you are not specifically referring the CarMD device regarding this situation, but the many tuning devices being offered to improve performance and gas mileage available on the market. I cannot speak for products made by other companies, but can assure you that the handheld CarMD device is what would fall under the automotive diagnostic “code reader” category that retrieves codes from a PCM only. While the CarMD system provides substantially more information to our customers than does a typical code reader or scan tool thanks to CarMD’s robust database, the CarMD device itself complies with all guidelines set by the Federal government in accordance with on board diagnostic second generation (OBD-2) code readers and their functions. The CarMD device is not considered a bi-directional scan tool that can interface with a vehicle’s PCM by tapping into the modules that affect various systems and make changes to fuel curves, transmission shift points, etc.

    Using the CarMD device would be like standing in a window and viewing what is displayed inside the store – you can see it but cannot touch it unless you go inside. CarMD then uses the codes found in a vehicle’s computer, reviews them through our extensive repair database and offers an accurate “most likely fix” based on expert input from our network of thousands of ASE-certified technicians. At no time does the CarMD device make any changes or have any chance to damage the PCM in any vehicle. Finally we get the power for the tool from the battery of the vehicle by tapping into the vehicle’s accessory power system not directly from the PCM.

    I hope this will assure you that our device is not designed to make any changes in any vehicle’s computer and cannot “scramble” the PCM in any vehicle by design. Its only purpose is to retrieve codes from the PCM and use that data to offer a most likely repair and associated parts/labor costs when you have a “Check Engine” light on so you as a vehicle owner can make an educated decision about getting it repaired right the first time.

    CarMD Technical Service Department

  • By j c rain, February 14, 2013 @ 2:14 pm

    does the a b s system have any connection with the transmission ??

  • By Ivy from CarMD, April 9, 2013 @ 7:08 pm

    Hi j c rain

    We just sent you a message through email and we will have our technical support team contact you shortly. Thank you!

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