by Jim Kerr
In 1996, vehicles changed dramatically, yet most consumers were not aware
of anything other than new styling. The big change was in the way
vehicle diagnostics were performed. It is commonly called OBD2.
OBD2 is the acronym for On Board Diagnostics Level 2 and it is applied
to the 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 by 1994.
OBD2 legislation affected every part of the emissions systems on
vehicles. A common terminology was adopted. Repair manuals had to be
presented in a specific arrangement. The position of the diagnostic
electrical connector on the vehicle was standardised. And the
information presented to automotive technicians by the vehicle’s
computer was expanded.
One of the standards was the way codes are to be displayed. OBD2 codes
follow the Society of Automotive Engineers (SAE) standard J2012. 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. Some problems will cause the MIL to
illuminate after only one test fails. Other emissions tests must fail
twice consecutively before the MIL comes on. If the problem is
intermittent, then the light may never come on, but a code will still be
stored in the vehicle’s computer. Non-emissions problems can also set
codes, but they do not turn on the MIL warning indicator.
Emissions related OBD2 codes can be read with a handheld computer called
a scanner or scan tool. Automotive technicians use a scanner extensively
when diagnosing modern computerised cars. Some automobile manufacturers
such as Chrysler and General Motors allow almost all the computer data
to be read by any scanner. Other manufacturers only allow emissions
related codes to be displayed by scanners unless you have the “factory”
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 computerised systems: B for body, C
for chassis, and U for computer communications networks.
The second digit is either the number 0 or 1. Generic codes (the same
code used by all manufacturers) are indicated by a 0. 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
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 may just be a
waste of money. Knowing the code is only the starting point to help you
find what area of the manual to begin your diagnosis. When the problem
is repaired, the codes can be cleared with the scan tool, or after the
vehicle has run the diagnostics through 40 vehicle warm up cycles
without a failure! The MIL light turns off after three successful
emissions tests by the vehicle’s computer, or it can also be reset using
the scan tool.
By the way, code P0134 can be set during extremely high speed driving at
wide-open throttle because the oxygen sensor is always showing a rich
air fuel mixture then. Other vehicle operating conditions are also
stored when the code is set, and a technician can display all the
information, including the vehicle speed when the problem occurred. This
has caused trouble on more than one occasion when a teenaged child has
borrowed the family vehicle. Imagine what the parents say when they find
out what vehicle speed the code was set at!