The EVAP system uses a Fuel Tank Pressure Sensor (located on top of the fuel tank sender unit.) in order to detect when the EVAP system is purging. The Fuel Tank Pressure Sensor detects the flow from the engine through the EVAP Canister Purge Solenoid Valve. When the EVAP system is not purging, the fuel tank pressure sensor will detect no change in fuel tank vacuum. When the EVAP System is purging, the Fuel Tank Pressure Sensor will detect a vacuum in the fuel tank. A change in Fuel Tank Pressure will be displayed on the scan tool.

If any of the following EVAP malfunctions are present:

One of the following DTCs will set:

If any of these DTCs are set, go to the DTC table for diagnosis.

The misfire monitor diagnostic is based on the crankshaft rotational variations, or reference period. The control module determines the crankshaft rotational velocity using the crankshaft position (CKP) sensor and the camshaft position (CMP) sensor. When a cylinder misfires, the crankshaft slows down momentarily. By monitoring the crankshaft and camshaft position sensor signals, the control module can calculate when a misfire occurs.

For a non-catalyst damaging misfire, the diagnostic is required to monitor a misfire present for between 1000-3200 engine revolutions.

For a catalyst damage misfire, the diagnostic responds to the misfire within 200 engine revolutions.

Rough roads may cause a false misfire detection. A rough road applies torque to the drive wheels and the drive train. This torque can intermittently decrease the crankshaft rotational velocity. The control module detects this as a false misfire.

On the automatic transmission equipped vehicles, the torque converter clutch (TCC) will disable whenever a misfire is detected. Disabling the TCC isolates the engine from the rest of the drive line and minimizes the effect of the drive wheel inputs on the crankshaft rotation.

When the TCC has disabled as a result of misfire detection, the TCC will re-enable after approximately 3200 engine revolutions if no misfire is detected. The TCC remains disabled whenever the misfire is detected, with or without a DTC set. This allows the misfire diagnostic to re-evaluate the system.

During a transmission high temperature condition, the misfire diagnostic will disable and the TCC will operate normally. This avoids further increasing the temperature of the transmission.

Whenever a cylinder misfires, the misfire diagnostic counts the misfires. Then the misfire diagnostic notes the crankshaft position at the time 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 indicate the number of firing events out of the last 200 cylinder firing events which were misfires. The misfire current counters display real time data without a misfire DTC stored. The misfire history counters indicate the total number of cylinder firing events which were misfires. The misfire history counters display 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.

Use Techline™ equipment to monitor misfire counter data on OB  ll 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 numbers 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.

The misfire counter information is located in the specific eng. menu, misfire data sub-menu 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. Possible malfunctions include the following conditions:

This system monitors the averages of short and long term FT 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 FT values and long term FT 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 diagnostic trouble code (DTC) will set.

In order to meet OBD ll 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 the vehicle operates fine at other times. No fuel trim DTC would set (although an engine idle speed DTC or O2S 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, including other DTCs stored, rich or lean condition, etc., when diagnosing a fuel trim fault.

The 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 malfunction indicator lamp (MIL) is a list of components, features or functions that could fall under this requirement.

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, for example, a throttle position (TP) sensor that indicates high TP at low engine loads or MAP voltage. The input components may include but are not limited to the following sensors:

In addition to the circuit continuity and rationality check, the ECT sensor is monitored for the ability to achieve a steady state temperature in order to enable a closed loop fuel control.

The output components respond 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:

Important: Not all vehicles have these components:

The control module harness electrically connects the control module to the various solenoids, switches, and sensors in the vehicle engine room and passenger compartment.

Replace the wire harnesses with the proper part number replacement. When splicing signal wires into a harness, use the wiring that has high temperature insulation.

Consider the low amperage and voltage levels utilized in the Powertrain control systems. Make the best possible bond at all splices. Use rosin-core solder in these areas.

Molded-on connectors require complete replacement of the connector. Splice a new connector into the harness. Replacement connectors and terminals are listed in Group 8.965 in the Standard Parts Catalog.

For wiring repair, refer to Wiring Repairs in Wiring Systems.

In order to prevent shorting between opposite terminals, use care when probing a connector and when replacing terminals. Damage to the components could result.

Always use jumper wires between connectors for circuit checking.

Never probe through the Weather-Pack seals.

Use the Tachometer Adapter J 35812, or the equivalent, which provides a convenient connection to the tachometer lead. The J 35616-A Connector Test Adapter Kit , or the equivalent, contains an assortment of flexible connectors used in order to probe the terminals during the diagnosis. The fuse remover and the BT-8616 test tool, or the equivalent, is used for removing a fuse and to adapt the fuse holder to a meter for diagnosis.

Open circuits are often difficult to locate by sight because oxidation or terminal misalignment are hidden by the connectors. Merely wiggling a connector on a sensor or in the wiring harness may temporarily correct the open circuit. Oxidized or loose connections may cause intermittent problems.

Be certain the type of connector and terminal before making any connector or terminal repair. Weather-Pack and Com-Pack III terminals look similar, but are serviced differently.

Powertrain Control Module (PCM) Serial Data Communication

U.S. Federal regulations require that all automobile manufacturers establish a common communications system. This vehicle utilizes the Class 2 communications system. Each bit of information can have one of two lengths, long or short. This allows the vehicle wiring to be reduced by the transmission and reception of the multiple signals over a single wire. The messages which are carried on Class 2 data streams are also prioritized. In other words, if two messages attempt to establish communications on the data line at the same time, only the message with the higher priority will continue. The device with the lower priority message must wait. The most significant result of this regulation is that the regulation provides the scan tool manufacturers with the capability of accessing the data from any make or model vehicle sold in the United States.

The Diagnostic Executive is a unique segment of the software which is designed to coordinate and prioritize the diagnostic procedures as well as define the protocol for recording and displaying their results. The Diagnostic Executive has the following main responsibilities:

A diagnostic test is a series of steps which has a beginning and an 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:

When a diagnostic test reports a fail result, the diagnostic executive records the following data:

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 a key ON and key OFF cycle in which all the enabling criteria for a given diagnostic have been met allowing the diagnostic to run. The requirements for trips vary as they may involve items of an unrelated nature, such as 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.

A warm-up cycle consists of an engine start-up and vehicle operation such that the coolant temperature has risen more than 4°C (40°F) from the start-up temperature and reached a minimum engine coolant temperature of 71°C (160°F). If this condition is not met during the ignition cycle, the diagnostic may not run.

The diagnostic tables and the functional checks are designed in order to locate a poor circuit or a malfunctioning component through a process of logical decisions. The tables are prepared with the assumption that the vehicle functioned correctly at the time of assembly and that there are no multiple faults present.

There is a continuous self-diagnosis on certain control functions. This diagnostic capability is complemented by the diagnostic procedures which are 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 will set and the malfunction indicator lamp (MIL) will illuminate on some applications.

The malfunction indicator lamp (MIL) is on the instrument panel. The MIL has the following functions:

When the MIL remains ON while the engine is running, or when a malfunction is suspected due to a driveability or emissions problem, perform an On-Board Diagnostic (OBD) System Check. The procedures for these checks are given in engine controls. These checks expose faults which the technician may not detect if other diagnostics are performed first.

The diagnostic executive must acknowledge when all the emissions related diagnostic tests have reported a pass or fail condition since the last ignition cycle. Each diagnostic test is separated into 4 types:

When a type A diagnostic test reports a failure, the diagnostic executive immediately requests to have the MIL turn ON for that diagnostic test. When a type B diagnostic test reports a failure during 2 consecutive trips, the diagnostic executive turns on the MIL for that diagnostic test. The diagnostic executive has the option of turning the MIL OFF when the diagnostic test which caused the MIL to illuminate the passes for 3 consecutive trips. In the case of misfire or fuel trim malfunctions, there are additional requirements as follows:

When the diagnostic executive requests the service light to be turned ON or a type C diagnostic fault is reported, a history DTC is also recorded for the diagnostic test. The provision for clearing a history DTC for any diagnostic tests requires 40 subsequent warm-up cycles during which no diagnostic tests have reported a fail, a battery disconnect, or a scan tool clear info command.

Unique to the misfire diagnostic, the diagnostic executive has the capability of alerting the driver of potentially damaging levels of misfire. If a misfire condition exists that could potentially damage the catalytic converter, the diagnostic executive will command the MIL to flash at a rate of once per second during the times that the catalyst damaging misfire condition is present.

Misfire and fuel trim malfunctions are special cases of type B diagnostics. Each time a fuel trim malfunction is detected, the engine load, the engine speed, and the 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 fuel trim malfunction occurs during 2 consecutive trips, the diagnostic executive treats the failure as a normal type B diagnostic. The diagnostic executive does not use the stored conditions. However, if a fuel trim malfunction occurs on 2 non-consecutive trips, the stored conditions are compared with the current conditions. The MIL will then illuminate under the following conditions:

Government regulations require that the engine operating conditions are to be captured whenever the MIL is illuminated. The data that is 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 failure records 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:

Freeze Frame data can only be overwritten with the data associated with a misfire or a fuel trim malfunction. The data from these faults take precedence over data that is associated with any other fault. The Freeze Frame data will not be erased unless the associated history DTC is cleared.

In the case of an intermittent fault, the malfunction indicator lamp (MIL) may illuminate and then after 3 trips turn OFF. However, the corresponding diagnostic trouble code will store in the memory. When unexpected diagnostic trouble codes appear, check for an intermittent malfunction.

The provision for communicating with the control module is a data link connector (DLC). The DLC is usually located under the instrument panel. The DLC is used in order to connect to a scan tool. Some common uses of the scan tool are listed below:

The control module has a learning ability which allows the control module to make corrections for minor variations in the fuel system in order to improve driveability. Whenever the battery cable is disconnected, the learning process resets.

The driver may note a change in vehicle performance. In order to allow the PCM to re-learn to drive the vehicle at part throttle with moderate acceleration. The vehicle may also operate at idle conditions until the normal performance returns.

Some vehicles allow the reprogramming of the control module without removal from the vehicle. This provides a flexible and a cost-effective method of making changes in software and calibrations.

Refer to the latest Techline information on reprogramming or flashing procedures.

Verification of the vehicle repair will be more comprehensive for vehicles with OBD II system diagnostics. Following a repair, the technician should perform the following steps:

1. Review the fail records and the Freeze Fame data for the DTC which was diagnosed. Record the fail records or Freeze Fame data. The Freeze Frame data will only store for an A or B type diagnostic.
2. Clear the DTCs.
3. Operate the vehicle within the conditions noted in the fail records or the Freeze Frame data.
4. Monitor the DTC status information for the specific DTC which has been diagnosed until the diagnostic test associated with that DTC runs.

Following these steps are very important in verifying repairs on the OBD II systems. Failure to follow these steps could result in an unnecessary repair.

Use a diagnostic scan tool in order to read the diagnostic trouble codes. Failure to follow this step could result in unnecessary repairs.

In order to clear Diagnostic Trouble Codes (DTCs), use the diagnostic scan tool clear DTCs or clear info function. When clearing DTCs follow the instructions supplied by the tool manufacturer. When a scan tool is not available, disconnecting one of the following sources for at least thirty (30) seconds can also clear the DTCs:

Notice: Turn off the ignition key when disconnecting or reconnecting battery power in order to prevent system damage.

Disconnecting the negative battery cable may result in the loss of other on-board memory data, such as preset radio tuning.

The OBD II vehicles have three options available in the scan tool DTC mode in order to display the enhanced information available. A description of the new modes, the DTC Info and the Specific DTC, follows. After selecting the DTC, the following menu appears:

The following is a brief description of each of the sub menus in the DTC Info and the Specific DTC. The order in which they appear here is alphabetical and not necessarily the way they will appear on the scan tool.

Use the DTC Info mode in order to search for a specific type of stored DTC information. There are seven choices. The electronic service information may instruct the technician to test for DTCs in a certain manner. Always follow the published service procedures.

In order to get a complete description of any status, press the Enter key before pressing the desired F-key. For example, pressing enter, then an F key will display a definition of the abbreviated scan tool status.

This selection displays any DTCs that have not run during the current ignition cycle or have reported a test failure during this ignition up to a maximum of 33 DTCs. The DTC tests which run and pass removes that DTC number from the scan tool screen.

This selection displays all of the DTCs that have failed during the present ignition cycle.

This selection displays only the DTCs that are stored to the history memory of the control module. The history memory will not display the Type B DTCs that have not requested the MIL. The history memory will display all of the type A and B DTCs that have requested the MIL and have failed within the last 40 warm-up cycles. In addition, the history memory will display all of the type C DTCs that have failed within the last 40 warm-up cycles.

This selection displays only the DTCs which have failed during the last time that the test ran. The last test may have ran during a previous ignition cycle if the a type A or B DTC is displayed. For type C DTCs, the last failure must have occurred during the current ignition cycle to appear as Last Test Fail.

This selection displays only the DTCs that are requesting the MIL. Type C DTCs cannot be displayed by using this option. This selection will report type B DTCs only after the MIL has been requested.

This option displays up to 33 DTCs that have not run since the DTCs were last cleared. Since any displayed DTCs have not run, their condition, passing or failing, is unknown.

This selection displays all of the active and history DTCs that have reported a test failure since the last time the DTCs were cleared. The DTCs that last failed over 40 warm-up cycles (before this option is selected) will not be displayed.

This mode is used in order to check the status of the individual diagnostic tests by the DTC number. This selection can be accessed if a DTC has passed or failed. Many OBD II DTC mode descriptions are possible because of the extensive amount of information that the Diagnostic Executive monitors regarding each test. Some of the many possible descriptions follow with a brief explanation.

This selection only allows the entry of the DTC numbers that are supported by the vehicle that is being tested. If an attempt is made to enter the DTC numbers for tests which the diagnostic executive does not recognize, the requested information will not be displayed correctly and the scan tool may display an error message. The same applies to using the DTC trigger option in the snapshot mode. If an invalid DTC is entered, the scan tool will not trigger.

For type A and B DTCs, this message will display during the subsequent ignition cycles until the test passes or the DTCs are cleared. For type C DTCs, this message clears whenever the ignition is cycled.

This message displayed indicates that the diagnostic test failed at least once within the last 40 warm-up cycles since the last time the control module cleared the DTCs.

This message displayed indicates that the diagnostic test has failed at least once during the current ignition cycle. This message will clear when the DTCs are cleared or the ignition is cycled.

This message displayed indicates that the DTC has stored to memory as a valid fault. A DTC displayed as a history fault does not necessarily mean that the fault is no longer present. The history description means that all of the conditions necessary for reporting a fault have met.

This message displayed indicates that the DTC is currently causing the MIL to turn ON. Remember that only type A and B DTCs can request the MIL. The MIL request cannot determine if the DTC fault conditions are currently being experienced. This is because the diagnostic executive requires up to 3 trips during which the diagnostic test passes to turn OFF the MIL.

This message displayed indicates that the selected diagnostic test has not run since the last time the DTCs were cleared. Therefore, the diagnostic test status, passing or failing, is unknown. After the DTCs are cleared, this message continues to be displayed until the diagnostic test runs.

This message displayed indicates that the selected diagnostic test has not run this ignition cycle.

This message displayed indicates that the selected diagnostic test has the following items:

Whenever the indicated status of the vehicle is Test Ran and Passed after a repair verification, the vehicle is ready to be released to the customer.

If the indicated status of the vehicle is Failed This Ign after a repair verification, then the repair is incomplete. A further diagnosis is required.

Prior to repairing a vehicle, use the status information in order to evaluate the state of the diagnostic test and to help identify an intermittent problem. The technician can conclude that although the MIL is illuminated, the fault condition that caused the code to set is not present. An intermittent condition must be the cause.