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.
Mike Allen is a longtime automotive expert and journalist. He owns and publishes SaturdayMechanic.com, 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.