Description of Common OBD II Terms
The 1998 Chevrolet Tracker conforms to all OBD II mandated emission regulations. The Powertrain Control Module (PCM) is responsible for monitoring and modifying engine controls in order to meet all OBD II requirements. Certain minimum criteria for how the PCM monitors and diagnoses the emission system is specified in the OBD II regulations. The following information describes the common operations and terms of the OBD II regulations that the Tracker PCM is designed to adhere to.Diagnostic
The word diagnostic, when used as a noun, refers to any on-board test run by the vehicle[prime]s Powertrain Control Module (PCM). A diagnostic is simply a test run on a system or component in order to determine if the system or the component is operating according to specifications. The following list defines the major vehicle on-board diagnostics. Depending on the emission requirements in the area of vehicle sale, certain diagnostics listed below may not apply.
^ Misfire
^ Oxygen sensors
^ Oxygen sensor heaters
^ EGR
^ Catalyst monitoring
Enable Criteria
The term enable criteria is engineering language for the conditions necessary for a given diagnostic test to run. Each diagnostic has a specific list of conditions which must be met before the diagnostic will run. Enable criteria is another way of saying conditions required.
The enable criteria for each diagnostic is listed on the first page of the DTC description under the heading Conditions for Setting the DTC. The enable criteria varies with each diagnostic, and typically includes, but is not limited to the following items:
^ Engine Speed (RPM)
^ Vehicle Speed (VSS)
^ Engine Coolant Temperature (ECT)
^ Mass Air Flow (MAF)
^ Barometric Pressure (BARO)
^ Intake Air Temperature (IAT)
^ Throttle Position (TP)
^ High Canister Purge
^ Fuel-trim
^ Torque Converter Clutch (TCC) status
Trip
Technically, a trip is a key on-run-key off cycle in which all the enable criteria for a given diagnostic are met, allowing the diagnostic to run. Unfortunately, this concept is not quite that simple. A trip is official when all the enable criteria for a given diagnostic are met. Because the enable criteria vary from one diagnostic to another, the definition of a trip varies as well. Some diagnostics are run when the vehicle is at operating temperature. Some diagnostics are run when the vehicle first starts up. Some diagnostics require that the vehicle is cruising at a steady highway speed. Some diagnostics run only when the vehicle is at idle. Some diagnostics function with the torque converter clutch disabled. Some diagnostics run only immediately following a cold engine start-up.
A trip then, is defined as a key on-run-key off cycle in which the vehicle was operated in such a way as to satisfy the enable criteria for a given diagnostic. This diagnostic will consider this cycle to be one trip. However, another diagnostic with a different set of enable criteria (which were not met) during this driving event, would not consider it a trip. No trip will occur for that particular diagnostic until the vehicle is driven in such a way as to meet all the enable criteria.
Passive and Active Diagnostic Tests
A passive test is a diagnostic test which simply monitors a vehicle system or a vehicle component. An active test actually takes some sort of action when the performing diagnostic functions. An active test is often in response to a failed passive test. For example, the Exhaust Gas Recirculation (EGR) diagnostic active test may force the EGR valve open during a closed throttle deceleration maneuver. Or the EGR diagnostic active test may force the EGR valve closed during a period of steady speed driving. Either action should result in a change in the manifold pressure.
Warm-Up Cycle
A warm-up cycle means that the engine temperature must reach a minimum of 70° C (160° F) and rise at least 22° C (40° F) over the course of a trip.
Freeze Frame
Freeze Frame is an element of the Diagnostic Management System that stores various vehicle information at the moment an emissions-related fault is stored in the PCM memory. Freeze Frame information is also stored when the MIL is commanded On. Freeze Frame data is useful in identifying the cause of an emission-related fault. Freeze Frame data is accessed by the scan tool and can be saved with the Capture Data feature. Refer to Storing And Erasing the Freeze Frame Data in Powertrain Control Module Diagnosis for more detailed information.
The PCM can store up to 4 frames of Freeze Frame data. The 1st frame stores data of the fault that was detected first. The data from the 1st detected fault is also stored in the 2nd frame of Freeze Frame data. The data stored in the 2nd frame of Freeze Frame data is permanent, and will not change when a new fault is detected. The 1st frame of Freeze Frame data will remain unchanged unless a fault of a higher priority occurs. A misfire fault (DTC P0300-P0304) or a fuel trim fault (DTC P0171 and P0172) will replace the data in the 1st frame of Freeze Frame data because these DTCs have a higher priority under OBD 2 rules. The 2nd through 4th frames of Freeze Frame data will store fault data in the order that the faults were detected regardless of the fault's priority. Utilizing the 4 frames of Freeze Frame data can provide information on the order in which the faults were first detected. The following table indicates how Freeze Frame data is stored when two or more faults are detected.
Diagnostic Information
The diagnostic tables and the functional checks are designed to locate a faulty circuit or component through a process of logical decisions. The diagnostic tables are prepared with the requirement that the vehicle functioned correctly at the time of assembly and that there are not multiple faults present.
There is a continuous self-diagnosis on certain control functions. This diagnostic capability is complemented by the diagnostic procedures contained in this service information. The language of communicating the source of the malfunction is a system of diagnostic trouble codes (DTCs). When a malfunction is detected by the powertrain control module a DTC is set and the Malfunction Indicator Lamp (MIL) lamp is illuminated.
Malfunction Indicator Lamp (MIL)
The MIL looks the same as the MIL you may be already familiar with (Service Engine Soon or Check Engine). OBD II regulations require that the MIL illuminate according to a strict set of guidelines. Under OBD II the MIL is turned on when the PCM detects a malfunction that will impact the vehicle emissions.
The MIL is controlled by the PCM. The MIL will be illuminated if an emissions-related diagnostic test indicates a malfunction has occurred. The MIL will stay illuminated until the system or the component passes the same test for three consecutive trips.
A vehicle that is experiencing a misfire malfunction that may cause damage to the Three-Way Catalytic (TWC) converter will flash the MIL once per second. The MIL will continue to flash once per second until the vehicle is outside of the speed and the load conditions that may cause damage to the TWC catalyst. The MIL will stop flashing and remain on steady once the vehicle is outside of the speed and the load conditions that may cause damage to the TWC catalyst.
Extinguishing the MIL
The PCM will turn Off the MIL after three consecutive trips that a test passed has been reported for the diagnostic test that originally caused the MIL to illuminate.
The Diagnostic Trouble Code (DTC) will remain in the PCM memory and the Freeze Frame record until forty (40) warm-up cycles have been completed and no faults exist.
An MIL that was illuminated by either a fuel-trim DTC or a misfire-related DTC has additional requirements that must be met in order to turn Off the MIL. The additional requirements are as follows:
^ The diagnostic tests that are passed must occur within 375 RPM of the RPM data stored at the time the last test failed.
^ The diagnostic tests that are passed must occur within 0 to 20% of the engine load that was stored at the time the last test tailed.
^ The diagnostic tests that are passed must occur at engine temperature conditions (warmed up or warming up) that are similar to those stored at the time the last test failed.
Meeting these requirements ensures that the fault which turned on the MIL has been corrected.
DTC Types
Each Diagnostic Trouble Code (DTC) is directly related to the passing or the failing of a diagnostic test(s). The PCM sets and stores the DTCs based on the failure of a test(s) during a trip(s). Certain tests must fail two (2) consecutive trips before the Malfunction Indicator Lamp (MIL) is illuminated and the DTC is set. There are four (4) types of DTCs.
The following are the characteristics of the four types of DTCs:
^ Type A
- The engine control system fault is emissions related.
- The MIL illuminates on the first trip with a failure.
- A History DTC is stored on the first trip with a failure.
- A Freeze Frame record is stored on the first trip with a failure (if empty).
^ Type B
- The engine control system fault is emissions related.
- The DTC and the MIL are armed after one (1) trip with a failure.
- The DTC and the MIL are disarmed after one (1) trip with a pass.
- The MIL illuminates on the second consecutive trip with a failure.
- A History DTC is stored on the second consecutive trip with a failure (the DTC will be armed after the first failure).
- A Freeze Frame record is stored on the second consecutive trip with a failure (if empty).
- Some special conditions apply to misfire and fuel trim DTCs.
^ Type C (if the vehicle is so equipped)
- Non-emission related
- Requests the illumination of the Service Lamp or the service message on the Driver Information Center (DIC) on the first trip with a failure.
- A History DTC is stored on the first trip with a failure.
- The Failure Record is updated each time the diagnostic test fails.
^ Type D (Type D DTCs are non-emissions related and are not utilized on certain vehicle applications).
- Non-emissions related
- Does not request illumination of any warning lamp.
- A History DTC is stored on the first trip with a failure.
- A Freeze Frame record is not stored with any test failures.
Special Cases Of Type B Diagnostic Tests
The PCM has the capability of alerting the vehicle operator to potentially damaging levels of misfire. A misfire condition that could potentially damage the catalytic converter as a result of high misfire levels will command the PCM to flash the MIL at a rate of once per second. This once per second flashing of the MIL will occur during the entire time that the catalyst damaging misfire condition is present. This flashing of the MIL is unique to the misfire diagnostic.