What Will a Car Diagnostic Tell You?

by Robert Moore

The computer in your vehicle is in constant contact with all of the sensors and most of the electronic devices in your vehicle. Anytime the computer detects an abnormal sensor reading, or inoperative sensor, it will log the error code and turn on your check engine light. This system -- known as on-board diagnostics II -- was a huge improvement over the original diagnostic systems used in vehicles made before 1996, but it still takes some skill and knowledge to accurately diagnose a problem.

What You Get From a Diagnostic

Running a diagnostic on your vehicle involves plugging a scanner into your vehicle's OBD-II port. Some models built before 1996 had a diagnostic system, but it was limited and varied greatly from manufacturer to manufacturer. The OBD-II system is standard on all vehicles sold in the United States after 1995. When you run codes on an OBD-II system, you'll receive one or more trouble code like "P0301" or "B-1402." OBD-I systems normally blink the check engine light on the dash to display a numerical code. OBD-I code meanings vary by manufacturer, while most codes for the OBD-II system are the same across manufacturers, except for some manufacturer-specific codes.


Trouble codes that start with "P" are powertrain trouble codes, trouble codes that start with "B" are body trouble codes and codes starting with "C" are chassis trouble codes.

The Translation

If you need to translate trouble codes for a pre-1996 vehicle made by any company other than GM, contact your car's manufacturer for an accurate list. To see a list of trouble codes for pre-1996 GM vehicles, visit Troublecodes.net. To translate trouble codes from any 1996 or newer vehicle in the United States, visit OBD-Codes.com.

The Hard Part

Trouble codes aren't definitive when it comes to diagnosis. For instance: A P0301 code means oxygen sensor one in bank one has a circuit malfunction, indicating the computer has received an unexpected signal voltage from the sensor. Wiring issues, a faulty oxygen sensor, a problem with the computer itself or incorrect air-to-fuel mixture can all cause a P0301 code. Just an incorrect air-to-fuel mixture has several causes like a vacuum leak, injector stuck open, leaky intake gasket or even a plugged air filter.

As another example a B1402 trouble code on Chrysler-built vehicles indicates a short to power in the circuit for the front, left speaker. On Ford built vehicles it indicates a fault in the circuit for the driver's power window switch. Trouble codes can also vary by manufacturer. The computer can't give you a complete diagnosis yet, but it does point you in the right direction.

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