Product Review - Topdon 309 CAN OBDII Code Reader

A blue card code reader with its plug

The TD 309 CAN OBDII/EOBD Code Reader is a nifty, simple device that can talk to your car’s computer. If your dashboard shows you an alert you don’t understand, this reader can tell you the underlying issue by identifying the “code” stored in your car’s system. It can also clear those codes, turn off your “Check Engine” indicator, and access a few other details about your car, like your Vehicle Identification Number (VIN).

Almost all cars made after 1996 include a simple, standardized system for recording and sharing information about the state of the vehicle (this feature is separate from any more complex technology that may be specific to the model). In the United States, these are called On-Board Diagnostic systems (OBDs), and the national standard, adopted in 1996, is called OBD-II (in Europe, it’s “EOBD” for European Onboard Diagnostics). OBD-II replaced several proprietary systems specific to manufacturers, and was the technical successor to OBD-I, a legal standard imposed only in California.

The OBD Controller Area Network (CAN) can be accessed by plugging a code reader into the Data Link Connector (DLC), a port with two rows of pin receivers that by regulation must be located within two feet of the steering wheel. Through the DLC, a reader can get information on the car’s system, including any Diagnostic Trouble Codes (DTCs).

These codes are all composed of four numbers preceded by one letter (B for body, C for chassis, U for network, and P for powertrain - aka engine and transmission), and there are hundreds of them, so they can be used to identify a wide range of specific issues. Some manufacturers also have supplemental proprietary codes, which should still be accessible by any standard code reader.

The pin plug of a car code reader

The TD 309 is a lightweight gadget with a bright baby-boy-blue design, and a screen reminiscent of early generation iPods or old-school vertical Game Boys. It only has two buttons: Enter/Exit and Scroll, but the software it contains is straightforward enough that those are really all you need. All the menus are functionally circular, so you can keep pressing “Scroll” to go through them and back to the beginning.

From the homescreen, you can either scan for codes and access vehicle information or adjust the device settings (language, measurements, and contrast). You can choose German (“Deutsch”), Dutch, English, Spanish, or French, select either English or metric measurements, and adjust the darkness of the on-screen text to your preference.

The diagnostic menu is a little more complex. From there, you can read or erase codes, view a freeze frame or I/M readiness, and access vehicle information (VIN, calibration ID, and calibration verification number). A freeze frame is a complete statistical picture of engine conditions in the moment of an emissions fault. I/M readiness refers to a set of standard ongoing measurements related to emissions. These are the tools used to perform smog tests on your vehicle.

diagnostic menu on a car code reader

Conveniently, the TD 309 draws power from your car when it’s plugged in, and turns on automatically when you connect it to your DLC. The chord is non-detatchable, so it can’t get lost, and at about two feet, it’s just long enough to allow you to hold the reader fairly easily while sitting in the driver’s seat. Plugging it in can be a little tricky, depending on where the DLC is in your car. We tested it on a 2007 Toyota Yaris and a 2002 Toyota 4Runner, and found it easiest to connect to both cars while kneeling outside the driver side door.

The TD 309 comes with a fairly comprehensive and clearly written instruction manual, which should be able to clear up any operational questions you encounter.

The bottom line is that if you're looking for a device that reads car codes so you can do some home auto repair and save a trip to the mechanic, this simple, handy product might be all you need. It doesn't have the elegance of some more advanced devices, but it covers the basics comprehensively. And at a price point of $20 as of this writing, it's far cheaper than many other readers on the market, while offering most of the core functionality delivered by those more expensive devices.

Topdon is a Chinese company with offices in several European nations. They sell a range of automotive diagnostic tools, from straightforward consumer products like code readers and battery testers to high-end touch screen devices for professional mechanics.