Driver finding coding system difficult to crack

Hey there, time traveller!
This article was published 27/07/2007 (6686 days ago), so information in it may no longer be current.

QUESTION — I have four vehicles in my family and do most the repair work myself except for transmissions. Recently, I have got into the OBD2 codes and find difficulty in understanding what my OBD2 scanner is displaying to me. From what I have read, OBD2 codes follow the Society of Automotive Engineers (SAE) standard J2012. Each part of the code shows some information. None of the manuals I have break them out. I may be wrong but I suspect one can only get the last two series of fault codes from special scanners or programs. Do you have any ideas on where I can get information on the last two code digits and what they indicate?

ANSWER — OBD2 is the acronym for On Board Diagnostics Level 2 and it a way the vehicle computer tests emission components and system operation. The standard was legislated in the United States to be applied for all 1996 and newer passenger cars and 1998 and newer trucks. Most Canadian vehicles, including trucks, met the U.S. OBD2 emissions standards by 1996 and a few were even compliant in 1994.

One of the standards was the way codes are to be displayed. If an emission related problem occurs on the vehicle, the MIL (malfunction indicator light) in the instrument panel comes on and a five digit code is stored in the vehicle computer. Non-emissions problems can also set codes, but they do not turn on the MIL warning indicator.

OBD2 codes are read with a handheld computer called a scanner or scan tool. Automotive technicians use a scanner extensively when diagnosing modern computerized cars. Some automobile manufacturers only allow emissions related codes to be displayed by scanners unless you have the "factory" scan tool. Others allow all codes to be show by any scanner.

Each part of the five digit code tells something about the problem. For example, in code P0134, the "P" indicates it is a powertrain code. Different letters are used for other computerized systems: B for body, C for chassis, and U for computer communications networks.

The second digit is either the number 0 or 1. A 0 indicates a generic code (the same code is used by all manufacturers). The number 1 indicates the code is specific to that manufacturer. The third digit may indicate a system such as the ignition system, fuel system, or transmission control system, while the last two digits represent a specific code. Code P0134 is a generic powertrain code and it would indicate the same problem in all manufacturers’ vehicles. The last two digits represent a code for the oxygen sensor in the fuel injection system.

Almost any 1996 and newer shop manual should give you the identification for the code, but diagnosing the fault is not always easy. For example, code P0134 is for an oxygen sensor that shows insufficient activity. Normally the oxygen sensor voltage is changing rapidly. The problem could be the oxygen sensor, but it could also be another problem such as a vacuum leak, engine misfire, or leaking injector. All of these problems can cause the oxygen sensor to become inactive.

Replacing the oxygen sensor may repair the problem, or it could be a waste of money. Knowing the code is only the starting point to help you find what area to begin your diagnosis.

QUESTION — Do you have any tips on storing a recreational vehicle battery? All I know is to keep it off the concrete floor. Mama doesn’t like it in the basement, but is the garage floor OK?

ANSWER — To store your battery, first charge it up completely. Then store it in a dry cold location. Your garage sounds like the ideal place. Keeping the battery cold slows down the chemical reaction and the battery keeps a charge much longer. If we could store them in a freezer, that would be perfect but extremely impractical! Put a trickle charger on the battery for a day or two every three months and your battery should be as good in the spring as it was when you stored it.

The reason for keeping a battery off concrete is to keep the battery dry. The concrete will become damp beneath the battery and the high humidity around the battery lets it discharge faster. Put the battery on a piece of wood or on a shelf where it can’t be knocked over. The wood lets the moisture from the concrete evaporate so the battery stays charged.

Jim Kerr is an experienced mechanic, instructor of automotive technology and member of the Automobile Journalists’ Association of Canada. You can e-mail questions to Jim at the address below.

kerr.jim@sasktel.net