Basics of a Vehicle Diagnostic

Nearly all vehicles on the road today have an interior computer that’s in contact with the sensors and electronic devices in the vehicle. When your check engine light comes on, as often as not it’s due to an issue identified by this computer.

This whole system is generally known as “on-board diagnostics,” and running a “diagnostic” test by plugging this computer into a scanner is a common part of modern auto service. We provide diagnostics as part of our car service at Shadetree Automotive – let’s look at some basics on diagnostic codes and translations, plus what this information can do for you.

Basics and Codes

When you run a diagnostic, a scanner will be plugged into a port called the OBD-II port that’s standard in all vehicles sold in the United States after 1996. Vehicles from before this time may have had a diagnostic system, but it would have been very limited and would vary greatly between manufacturers.

When you run a diagnostic, you’ll receive codes that look like these: “P0301” or “B-1402.” These codes are standardized across manufacturers in most cases, with the exception of a few manufacturer-specific codes. Codes that begin with “P” are powertrain codes, codes that begin with “B” are body codes, and codes that begin with “C” are chassis trouble codes.


For translating vehicle codes from pre-1996, contact your car’s manufacturer for an accurate list. Translating codes for vehicles since then is much easier – you or your technician can simply use an easy OBD online resource.

The Tough Part

Trouble codes aren’t always definitive when diagnosing an issue, and that’s why you come to the experts. A P0301 code, for instance, means an oxygen sensor in bank one has a malfunction, indicating the computer has received unexpected voltage – but this could have been caused by wiring issues, a faulty oxygen sensor, a problem with the computer or even an incorrect air-to-fuel mixture. The diagnosis will often point you in the right direction, but you’ll commonly need an experienced technician to pinpoint the problem.

Want to learn more about diagnostics, or interested in any of our other auto repair services? Speak to the pros at Shadetree Automotive today.