FAQ: OBD2 Codes

Many people are confused about the check engine light and why it comes on.

Myth 1: The trouble codes will tell you what sensor to replace. This is an expensive frame of mind. The Check Engine Light directs your attention to a problem. A trouble code description then directs you to a circuit or system. It will not tell you what sensor to replace. It is about as vague as stating that a book is in the automotive section of the library. You now know what section to go to, but no clue where to look yet. That is why there are flow charts or trouble code trees and diagrams to guide you through specific tests to determine what is the problem. Often it is a broken wire, loose connector or some other cause, than the sensor itself.

Myth 2: You can clear the codes by disconnecting the battery. This is true on pre-96 vehicles and very few, if any, OBD2 vehicles. Some folks will say, "I disconnected the battery for 15 minutes and the light went out, so it cleared the codes". No, it didn't. It may have reset the ECM and the light is no longer present, but the code is still there and if the problem has not been repaired, the light will come back on. The next time you have a problem, now you or the mechanic who is working on the vehicle are going to have to contend with that code as well as any other that is present.

Note: There are the rare cases where the manufacturer has made provisions for clearing the codes by disconnecting the battery or removing the ECM fuse. In most cases of post 96 vehicles, that is not the case, as most manufacturers have made the ECU/ECM/PCM retain the information, even in the event of battery voltage loss. In the largest percentage of the vehicles, unless specifically stated by the manufacturer, an OBD2 scantool is required to clear or reset the codes.

Myth 3: When the check engine light comes on, it always means you have to replace something. One of the first things that needs to be done when diagnosing the check engine light is to clear the trouble codes, road test the vehicle and then recheck the trouble codes. If the codes come back, then start with the lowest number code and go through the flow charts and diagrams.

The last Myth: The Check engine light means an O2 sensor problem. Anyone who has taken this to be true and has spent quite a bit of money on replacing oxygen sensors knows this is not true.

First, you will not know, nor will anyone else, what the problem is until you have the trouble codes. Even if it is an oxygen sensor code, alot of times there are other causes for the code to come about.

Vacuum leaks, poor fuel quality, low or high fuel pressure, a compression problem or a plugged catalytic converter could cause it. If the oxygen sensor is bad, then replacing the sensor still does not finish the repair. If an oxygen sensor failed, then there is a problem with the emissions of the engine. Usually when an oxygen sensor fails, it is because it has become contaminated. Contamination caused by an engine that is not running properly. So you will still need to determine where the originating problem came from. More often than not, there will be no trouble codes for that problem and it will not be evident without some specific tests.

OBD-II codes consist of a number of parts. Here is a sample code: P0171 Here is a breakdown of what each digit of the code means:

First Character - System

The first character identifies identifies the system related to the trouble code.

Second Digit - Code Type

The second digit identifies whether the code is a generic code (same on all OBD-II equpped vehicles), or a manufacturer specific code.

Third Digit - Sub-System

The third digit denotes the type of sub-system that pertains to the code

Fourth and Fifth Digits

These digits, along with the others, are variable, and relate to a particular problem. For example,a P0171 code means P0171 - System Too Lean (Bank 1).

Decoding resources on the web