The Hidden Risks of DIY Car Diagnostics

Peek under the steering wheel of your car, and you’ll likely see a chunky connector that looks a bit like the jaws of a plastic monster. This is your car’s OBD-II port (“OBD” stands for “onboard diagnostics”), the gateway to the computer that monitors and controls your vehicle’s operation.

While data access has existed on passenger vehicles in some form since the 1980s, the now-ubiquitous OBD-II port began in 1996 as part of a federal law designed to standardize automotive diagnostics. It’s been a mandated feature in all cars sold in the United States ever since.

Access to the information that’s available on a car’s computer requires a device—known variously as an OBD-II scanner or OBD-II code reader—that plugs into this port. While originally marketed to auto technicians, these scanners are now widely available to consumers.

Current models range from the $60 Gooloo DS200 to the $899 Innova 7111 and beyond, with myriad options in between. Some are quite basic, letting you read the error codes the engine’s sensors have generated. Others are more advanced, allowing you to actively test engine components. All of them let you find out what prompted that check engine light without taking your car to a mechanic.

The question is, should you use one yourself?

What is an OBD-II scanner?

OBD-II scanners are an invaluable part of every professional technician’s tool kit, but they’re also increasingly common in the garages of weekend warriors. Without one, there’s no way to understand what’s happening inside your vehicle and its hundreds of onboard sensors.

At its simplest, when you plug a scanner into the port, it will show you an error code—something like P0141. You can then look up that code online to find out what it means. (AAA has a lookup tool.) For example, if you plugged in P0141, you would learn that your engine is reporting an issue with its oxygen sensor heater circuit.

But it’s one thing to know why the check engine light came on. It’s another to know what to do with that information.

David Bennett, director of automotive growth and experience for AAA and a master certified auto technician, says that car owners often jump to conclusions when they get an error code translation. And that leap can get them into trouble.

“Once a technician has an error code, they will start down a diagnostic tree to understand the reason the code came on,” says Bennett. “A reference manual will walk them through a series of specific tests. For example: ‘Check the voltage here; if it’s out of specification, do this next.’”

Unfortunately, many do-it-yourselfers skip all of that. Instead, they translate the code, then use that information to make rash decisions. The error code says there’s a problem with the oxygen sensor? Some of those DIYers might replace the whole thing.

“They don’t understand that the issue involves something with that sensor’s subsystem, not necessarily that single component,” says Bennett. In other words, a single error code can be tripped by any one of several underlying problems.

Because of this, Bennett says that OBD-II code readers can quickly get tinkerers into trouble, convincing them to re­place components that are working just fine or making repairs that don’t need to be done. The translation of a code like P0457—EVAP system leak detected—might sound scary, but it usually just means the gas cap is loose. Having a code scanner on hand can be helpful, but only if you have the proper service manuals and the restraint to avoid making decisions without fully understanding the problem.

Another issue with code scanners, says Bennett, is that they can be used to clear codes and turn off warning lights, even if the underlying problem hasn’t been fixed. That can blind the user to serious problems that could become worse over time if they’re left unfixed. Bennett also notes that a check engine light can indicate problems with multiple systems; technicians know how to follow the process of diagnosing and correcting them in the right order. An amateur is less likely to understand those intricacies.

Do you need an OBD-II scanner?

So is there any reason for the average car owner to use a diagnostics code reader? “Not really,” says Bennett, “unless you want to tell the technician what the code is when you get to a repair facility, or you want to do some research online beforehand.” Even if you do tell your mechanic about an error code that you found with your own reader, they’ll still do a scan themselves to verify it.

“The bottom line is that this kind of information can be nice to have,” says Bennett, “as long as you don’t get yourself sideways with it and start buying parts you don’t need whenever a code pops up.”