TECHNICAL TERMSBaud Rate.....The speed at which bits of computer data are transmitted through the serial data stream or on the controller area network (CAN) bus.
Bus.....Another name for the basic circuit that carries data between all the modules in a vehicle. Also refered to as the Controller Area Network (CAN) bus.
CAN.....Controller Area Network, the bus circuit over which all the modules in the vehicle communicate with each other. Communication faults over the bus can cause unpredictable driveability problems and will set special fault codes.
Closed Loop.....An operating mode of the fuel feedback control system where the powertrain control module uses the rich/lean signal from the oxygen sensor to control the fuel mixture. Closed loop usually does not occur until the oxygen sensor is hot to produce a signal, and the engine has reached a certain temperature. The sooner the engine goes into closed loop operation, the lower are its emissions. A faulty coolant sensor, oxygen sensor or thermostat can prevent the engine from going into closed loop. This usually causes the engine to run rich and pollute.
Code Reader.....A type of diagnostic tool that plugs into the OBD II diagnostic connector and displays fault codes. A code reader can also be used to clear codes, and some will show OBD II readiness flags, but most are not capable of reading sensor data or other system information.
Downstream O2 sensor.....The oxygen sensor located behind (downstream) of the catalytic converter. This sensor is used primarily to monitor the operating efficiency of the catalyst, but may also be used for long term fuel trim.
rive Cycle.....When the engine is started, allowed to reach operating temperature and then shut down. It may or may not involve actually driving the vehicle.
Enable Criteria.....The set of conditions needed for a diagnostic test or OBD II monitor to run.
Fuel trim.....This is the fine-tuning of the air/fuel mixture. There is both short term fuel trim and long term fuel trim. Short term compensates for changing operating conditions. Long term compensates for engine wear and other variables.
Loaded Mode Testing.....A type of emissions testing that places a load on the vehicle to simulate driving conditions. This is done on a dynamometer while the vehicle is driven at various speeds. Loaded mode testing is used in both I/M 240 and ASM emissions testing.
Misfire Detection.....An OBD II monitor that detects misfire in one or more cylinders. Misfire may be caused by ignition, fuel or compression problems. If misfires exceed a certain rate, it sets a misfire code which corresponds to the cylinder(s) involved. If the misfire jumps around from cylinder to cylinder, OBD II sets a "random misfire" code (P0300) which mayb e the result of a vacuum leak, EGR leak or low fuel pressure.
Monitor.....A self-diagnostic check run by OBD II on a system or component.
Open Loop.....An operating mode of the fuel feedback control system where the fuel mixture is fixed (unchanging) and is predetermined by the computer. The engine remains in open loop after it is first started and until it reaches a certain temeprature and the oxygen sensor is hot enough to produce a good signal.
Readiness Code or Flag.....An indication that a particular OBD II monitor has run or has been completed. Emission test requirements requires that all by one or two readiness monitors be completed before a vehicle can pass an OBD II emissions test (note: this requirement varies by model year and state).
Scan Tool.....A type of diagnostic tool that plugs into the OBD II diagnostic conenctor to access system information including fault codes, history codes, sensor data and other information. Professional grade scan tools are also "bidirectional" and can be used to perform special tests. Some can also be used to reprogram the vehicle computer.
Trip.....Driving the vehicle long enough and under the right conditions so that all the OBD II system and component monitors have run at least once.
Upstream O2 sensor.....The oxygen sensor located in the exhaust manifold and ahead (upstream) of the catalytic converter. This sensor is used by the fuel feedback control system to monitor the relative richness or leanness of the air/fuel mixture for emissions, fuel economy and performance.
Warm-up Cycle.....Starting a cold engine and allowing it to warm up at least 40 degrees F (22 degrees C) and reach a minimum of 160 degrees F (70 degrees C).