On-Board Diagnostic II System
Misfire Diagnostics
Whenever a cylinder misfires, the misfire diagnostic counts the misfire and notes the crankshaft position at the time it the misfire occurred. These misfire counters are basically a file on each engine cylinder.
A current and a history misfire counter is maintained for each cylinder. The misfire current counters (Misfire Cur # 1 - 8) indicate the number of firing events out of the last 200 cylinder firing events which were misfires The misfire current counters displays real time data without a misfire DTC stored. The misfire history counters (Misfire Hist # 1 - 8) indicate the total number of cylinder firing events which were misfires. The misfire history counters displays 0 until the misfire diagnostic has failed and a DTC P0300 is set. Once the misfire DTC sets, the misfire history counters will be updated every 200 cylinder firing events.
The Misfire counters graphic illustrates how these misfire counters are maintained. If the misfire diagnostic reports a failure, the Diagnostic Executive reviews all of the misfire counters before reporting a DTC. This way, the Diagnostic Executive reports the most current information.
When crankshaft rotation is erratic, the control module detects a misfire condition. Because of this erratic condition, the data that is collected by the diagnostic can sometimes incorrectly identify which cylinder is misfiring. The Misfire Counters graphic shows there are misfires counted from more than one cylinder. Cylinder #1 has the majority of counted misfires. In this case, the Misfire Counters would identify cylinder #1 as the misfiring cylinder. The misfires in the other counters were just background noise caused by the erratic rotation of the crankshaft. If the number of accumulated is sufficient for the diagnostic to identify a true misfire, the diagnostic will set DTC P0300 Misfire Detected.
Use GM Techline equipment to monitor misfire counter data on OBDII compliant vehicles. Knowing which specific cylinders misfired can lead to the root cause, even when dealing with a multiple cylinder misfire. Using the information in the misfire counters, identify which cylinders are misfiring. If the counters indicate cylinders number 1 and 4 misfired, look for a circuit or component common to both cylinders number 1 and 4 such as an open ignition coil in an electronic ignition system.
Misfire counter information is located in the Specific Eng. menu, Misfire Data sub-menu of the of the data list.
The misfire diagnostic may indicate a fault due to a temporary fault not necessarily caused by a vehicle emission system malfunction. Examples include the following items:
^ Contaminated fuel
^ Running out of fuel
^ Fuel fouled spark plugs
^ Basic engine fault
Fuel Trim System Monitor Diagnostic Operation
This system monitors the averages of short-term and long-term fuel trim values. If these fuel trim values stay at their limits for a calibrated period of time, a malfunction is indicated. The fuel trim diagnostic compares the averages of short-term fuel trim values and long-term fuel trim values to rich and lean thresholds. If either value is within the thresholds, a pass is recorded. If either value is outside their thresholds, a rich or lean DTC will set.
In order to meet ODBII requirements, the control module uses weighted fuel trim cells in order to determine the need to set a fuel trim DTC. A fuel trim DTC can only be set if fuel trim counts in the weighted fuel trim cells exceed specifications. This means that the vehicle could have a fuel trim problem which is causing a concern under certain conditions (i.e. engine idle high due to a small vacuum leak or rough due to a large vacuum leak) while it operates fine at other times. No fuel trim DTC would set (although an engine idle speed DTC or HO2S DTC may set). Remember, use a scan tool in order to observe fuel trim counts while the problem is occurring.
Remember, a fuel trim DTC may be triggered by a list of vehicle faults. Make use of all information available (other DTCs stored, rich or lean condition, etc.) when diagnosing a fuel trim fault.
Comprehensive Component Monitor Diagnostic
Comprehensive component monitoring diagnostics are required to monitor emissions-related input and output Powertrain components. The CARB OBD II Comprehensive Component Monitoring List of Components Intended To Illuminate The MIL is a list of components, features or functions that could fall under this requirement.
Input Components
The Control Module monitors the input components for circuit continuity and out-of-range values. This includes performance checking. Performance checking refers to indicating a fault when the signal from a sensor does not seem reasonable (i.e. a Throttle Position (TP) sensor that indicates high throttle position at low engine loads or MAP voltage). The input components may include but are not limited to the following sensors:
^ The Vehicle Speed Sensor (VSS)
^ The Crankshaft Position (CKP) sensor
^ The Knock Sensor (KS)
^ The Throttle Position (TP) sensor
^ The Engine Coolant Temperature (ECT) sensor
^ The Camshaft Position (CMP) sensor
^ The Manifold Absolute Pressure (MAP) sensor
^ The Mass Air Flow (MAF) sensor In addition to the circuit continuity and rationality check, the ECT sensor is monitored for its ability to achieve a steady state temperature to enable closed loop fuel control.
Output Components
Diagnose the output components for the proper response to control module commands. Components where functional monitoring is not feasible will be monitored for circuit continuity and out-of-range values if applicable.
Output components to be monitored include, but are not limited to the following circuits:
^ The Idle Air Control (IAC) Motor
^ The Control module controlled EVAP Canister Purge Valve
^ The Electronic transmission controls
^ The A/C relay
^ The Cooling fan relay
^ The VSS output
^ The MIL control
^ The Cruise control inhibit
Components Intended to Illuminate MIL
The California Air Resources Board (CARB) OBDII program requires specific component malfunctions to illuminate the Malfunction Indicator Lamp (MIL). A list of these components is located in the accompanying image.
Important: Not all vehicles have these components.
The Diagnostic Executive
The Diagnostic Executive is a unique segment of software which is designed to coordinate and prioritize the diagnostic procedures as well as define the protocol for recording and displaying their results. The main responsibilities of the Diagnostic Executive are:
^ Monitoring the Diagnostic Test Enabling Conditions
^ Requesting the Malfunction Indicator Light (MIL)
^ Illuminating the MIL.
^ Recording Pending, Current, and History DTC(s)
^ Storing and Erasing Freeze Frame Data
^ Monitoring and Recording Test Status information.
On Board Diagnostic Tests
A diagnostic test is a series of steps which has a beginning and end. The result of which is a pass or fail reported to the Diagnostic Executive. When a diagnostic test reports a pass result, the Diagnostic Executive records the following data:
^ The diagnostic test has completed since the last ignition cycle.
^ The diagnostic test has passed during the current ignition cycle.
^ The fault identified by the diagnostic test is not currently active.
When a diagnostic test reports a fail result, the Diagnostic Executive records the following data:
^ The diagnostic test has completed since the last ignition
^ The fault identified by the diagnostic test is currently active.
^ The fault has been active during this ignition cycle.
^ The operating conditions at the time of the failure.
Trip
The ability for a diagnostic test to run depends largely upon whether or not a Trip has been completed. A Trip for a particular diagnostic is defined as vehicle operation, followed by an engine off period of duration and driving mode such that any particular diagnostic test has had sufficient time to complete at least once. The requirements for trips vary as they may involve items of an unrelated nature; driving style, length of trip, ambient temperature, etc. Some diagnostic tests run only once per trip (e.g. catalyst monitor) while others run continuously (e.g. misfire and fuel system monitors). If the proper enabling conditions are not met during that ignition cycle, the tests may not be complete or the test may not have run.
Warm-up Cycle
In addition, the execution of diagnostic tests may also be bound by conditions which must comprehend a Warm-up cycle. A Warm-up cycle consists of engine start-up and vehicle operation such that the coolant temperature has risen greater than a certain value (typically 40°F) from start-up temperature and reached a minimum temperature of 160°F. If this condition is not met during the ignition cycle, the diagnostic may not run.
Diagnostic Information
The diagnostic charts and functional checks are designed to locate a faulty circuit or component through a process of logical decisions. The charts 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 manual. The language of communicating the source of the malfunction is a system of diagnostic trouble codes. When a malfunction is detected by the control module, a diagnostic trouble code is set and the Malfunction Indicator Lamp (MIL) Service Engine Soon is illuminated on some applications.
Malfunction Indicator Lamp
The Malfunction Indicator Lamp (MIL) (Service Engine Soon) is on the instrument panel and has the following functions:
^ It informs the driver that a fault that affects vehicle emission levels has occurred and that the vehicle should be taken for service as soon as possible.
^ As a bulb and system check, the MIL (Service Engine Soon) will come ON with the key ON and the engine not running. When the engine is started, the MIL will turn OFF.
When the MIL remains ON while the engine is running, or when a malfunction is suspected due to a driveability or emissions problem, a Powertrain On-Board Diagnostic (OBD) System Check must be performed. This check will expose faults which may not be detected if other diagnostics are performed first.
MIL Requests and History Codes
(What happens when type A, B, C and D DTCs report failures and passes, and how the MIL responds. Includes criteria for turning the MIL ON and OFF.)
The Diagnostic Executive must be able to acknowledge when all emissions related diagnostic tests have reported a pass or fail condition since the last ignition cycle. Diagnostic tests are separated into different types:
^ A emissions related, turns ON the MIL,
^ B emissions related, turns ON the MIL if a fault is active for 2 consecutive driving cycles(except for misfire and fuel trim DTCs),
^ C non-emissions related, does not turn ON the MIL, but may turn ON a Service Lamp or message.
^ D non-emissions related, no notification given other than storing. D non-emissions related may not be utilized on certain vehicle applications.
When a type A diagnostic test reports a failure, the Diagnostic Executive initiates a request to have the MIL turn on for that diagnostic test. When a type B diagnostic test reports a second failure, the Diagnostic Executive initiates a request for that diagnostic test. A type D failure records the DTC but does not illuminate the MIL. The Diagnostic Executive has the option of turning the MIL OFF when three consecutive trips are recorded where the diagnostic system passes the test. In the case of misfire or fuel trim malfunctions, there is an additional requirement that the load conditions must be within 10%, the speed conditions must be within 375 rpm, and the coolant temperatures must be in the same calibratable high or low range at the time the diagnostic test last reported a failure. For a type C diagnostic test, the Diagnostic Executive will request that the message or Auxiliary Service Lamp (if so equipped) be turned off at the next ignition key-on cycle or test passing report.
When the Diagnostic Executive requests the MIL to be turned ON, a history DTC is also recorded for the diagnostic test. To clear a history DTC for most diagnostic tests requires 40 subsequent warm-up cycles during which no diagnostic tests have reported a fail (misfire and fuel trim DTCs require 80 cycles).
Special Cases Of Type B Diagnostic Tests
Unique to the misfire diagnostic, the Diagnostic Executive has the capability of alerting the vehicle operator to potentially damaging levels of misfire. If a misfire condition exists that could potentially damage the catalytic converter as a result of high misfire levels, the Diagnostic Executive will command the MIL to flash at a rate of once per second during the time that the catalyst damaging misfire condition is present.
Fuel trim and misfire are special cases of type B diagnostics. Each time a fuel trim or misfire malfunction is detected, engine load, engine speed, and engine coolant temperatures are recorded.
When the ignition is turned off, the last reported set of conditions remain stored. During subsequent ignition cycles, the stored conditions are used as a reference for similar conditions. If a malfunction occurs during two consecutive trips, the Diagnostic Executive treats the failure as a normal type B diagnostic, and does not use the stored conditions. However, if a malfunction occurs on two non-consecutive trips, the stored conditions are compared with the current conditions. The MIL will then illuminate under the following conditions:
^ When the engine load conditions are within 10% of the previous test that failed.
^ Engine speed is within 375 rpm, of the previous test that failed.
^ Engine coolant temperature is in the same range as the previous test that failed.
Storing And Erasing Freeze Frame Data
Government regulations require that engine operating conditions be captured whenever the MIL is illuminated. The data captured is called freeze frame data. The freeze frame data is very similar to a single record of operating conditions. Whenever the MIL is illuminated, the corresponding record of operating conditions is recorded to the freeze frame buffer.
Each time a diagnostic test reports a failure, the current engine operating conditions are recorded in the freeze frame buffer. A subsequent failure will update the recorded operating conditions. The following operating conditions for the diagnostic test which failed typically include the following parameters:
^ Air Fuel Ratio
^ Air Flow Rate
^ Fuel Trim
^ Engine Speed
^ Engine Load
^ Engine Coolant Temperature
^ Vehicle Speed
^ TP Angle
^ MAP/BARO
^ Injector Base Pulse Width
^ Loop Status
Freeze frame data can only be overwritten with data associated with a misfire or fuel trim malfunction. Data from these faults take precedence over data associated with any other fault. The freeze frame data will not be erased unless the associated history DTC is cleared.
Intermittent Malfunction Indicator Lamp
In the case of an intermittent fault, the MIL (Check Engine) may illuminate and then (after three trips) go OFF. However, the corresponding diagnostic trouble code will be stored in the memory. When unexpected diagnostic trouble codes appear, check for an intermittent malfunction.
A diagnostic trouble code may reset. Consult the "Diagnostic Aids" associated with the diagnostic trouble code. A physical inspection of the applicable sub-system most often will resolve the problem.