If you are reasonably certain that the problem is engine related, the first place to start is with the Powertrain On-Board Diagnostic (OBD) System Check.

Important: Do Not clear the DTCs unless a diagnostic procedure instructs you to do so.

While performing a diagnostic procedure, you may need to gain access to certain components. Read the On-Vehicle Service procedures for those components.

During the diagnosis or if a wiring problem is present, refer to Wiring Repairs in Wiring Systems. This section contains complete information on vehicle wiring and includes the following information:

After diagnosing a problem, in addition to repairing the discovered problem, perform the verification procedure which is the last few steps at the end of almost all of the diagnostic tables. Here you may be instructed to clear the DTCs. In order to clear the DTCs, refer to Clearing the DTCs later in this section. In order to verify a repair, you must duplicate the conditions when the customer's complaint occurred or if a DTC was set, you must duplicate the test conditions for that DTC (listed on the facing page of the trouble chart) and see if the DTC resets. If the DTC does not reset and the repair was tested under the proper conditions for the diagnostic to run, chances are good that the problem is repaired.

The diagnostic procedures in this section are designed to find and repair engine control module related problems. The general approach is to find the appropriate diagnosis for a problem by using the basic steps described below.

1. Understand the customer's complaint. Failure to understand the customer's complaint may lead to misdiagnosis. The technician must know whether the condition is present at all times, only under certain circumstances, or truly intermittent (random). This will assist the technician in duplicating and diagnosing the problem. Another reason the technician must understand the customer's complaint is so the technician may determine whether the complaint requires service or is normal vehicle operation. Diagnose a condition that is normal will waste time and may result in unnecessary service.
2. Are the Diagnostics working properly? Use the Powertrain OBD System Check. This is the starting point for all of the diagnostic procedures. Always begin here.
3. Are the DTCs displayed? If a DTC is identified by the diagnostics, the Powertrain OBD System Check will direct you to the appropriate chart within Engine System Diagnosis.
4. Is the customer's complaint related to a specific engine subsystem? If no related DTCs are set, the next quickest way to locate the problem is to narrow the problem down to a specific engine subsystem. If a specific subsystem can be pinpointed as the cause, the problem will be easier to diagnose. Engine subsystems are listed in Supplemental Information at the beginning of this section, along with the appropriate service manual section in order to reference for the descriptions and the diagnosis.

Diagnostic charts, wiring diagrams, and descriptive text direct you in performing specific tests in order to locate and repair problems. The diagnostic chart is a step by step procedure used to determine which circuit or component is the source of the problem. Wiring diagrams, text describing the system, test conditions, fault conditions, and actions taken when a DTC is set are located on the page facing the diagnostic chart.

Important: Never clear the Diagnostic Trouble Codes (DTCs) unless you are instructed to do so by the diagnostic procedure. Clearing the DTCs will erase the Freeze Frame buffer. Clearing the DTCs may also reset the Maintenance Inspection flags that are used for emission testing stations. If for any reason the ECM memory is cleared (by a battery disconnect, the use of a scan tool, or through a charging system malfunction), the diagnostics must be run in order to reset the flags.

Use a scan tool in order to read the diagnostic trouble codes.

Since the ECM can have a fault that affects only one circuit, following the diagnostic procedures in this section will help determine which circuit has the problem and where the problem is located.

If a diagnostic table indicates that the ECM or the ECM connections are the cause of the problem, but replacing the ECM does not correct the problem, check for the following conditions:

Important: Using the scan tool to clear the DTCs causes both the TCM and the ECM DTCs to clear, regardless of which system is being diagnosed.

Do Not clear the DTCs unless a diagnostic procedure instructs you to do so.

Clearing the DTCs also clears the Freeze Frame data. The scan tool has the capability to save any data that is stored in the Freeze Frame and then display that data at a later time. Once you have corrected and verified the problem, clear the DTCs so that any future service work is not needlessly confused.

Many of the ECM DTCs have complex test and failure conditions. Therefore, simply clearing the DTCs, exiting the diagnostics, and watching to see if the DTC sets again may not indicate whether a problem has been corrected. To verify that a repair is complete, you must look up the test conditions (on the facing page to the trouble chart) and duplicate those conditions. Then if the DTC does not set, chances are the problem has been repaired.

The ECM snapshot feature is designed to assist in the diagnosis of intermittent problems. When an ECM snapshot is taken, a predetermined subset of the ECM's data parameters at that time are stored. This data can be retrieved for use in diagnosing a problem. The scan tool can be set up to take a data snapshot when a specific DTC sets, if any DTC sets, or when the DTC is manually triggered with the scan tool.

The ECM snapshots may be useful in diagnosing intermittent engine problems. Stored data may be retrieved and used to determine if a particular input was incorrect when the problem occurred. Many of the ECM DTC trouble charts use the Freeze Frame information in order to pinpoint a problem if the problem is not current. For example, if a vehicle has an intermittent rough engine condition and a scan tool Snapshot is taken while the engine is running rough, the Snapshot will retrieve several data parameters, making it possible to detect an abnormal parameter. The ECT sensor, the IAT sensor, and the knock sensor parameters may be compared to the normal values. The values that are out of the normal range should be investigated. Also, the fuel injector pulse width or the spark advance can be retrieved with the Snapshot function. This data can reveal fueling or ignition irregularities that should be investigated. While this method may result in increased diagnosis time. this method also results in less actual repair time and fewer service comebacks.

The most important check is a visual/physical inspection. Often a thorough examination of a suspected system may uncover the cause of a problem. A Visual/Physical Inspection may take only a few minutes and can save you valuable time.

All engine diagnosis should begin with a thorough visual inspection. A visual inspection can often lead to repair of a simple problem without the use of charts. Check the following areas:

When diagnosing the engine, you will need to use the diagnostic procedures in this section or other engine sections. Most diagnostic procedures are in the form of tables. In the supporting text for each diagnostic table is a circuit diagram (in most cases), a circuit description, the conditions under which the ECM will run the diagnostic, the conditions that will cause the DTC to set, a description of the action taken (MIL operation, default actions)- any diagnostic aids, and test descriptions that elaborate on the certain steps in the table. Reading the supporting text will help you to understand the DTC or the condition and what the trouble chart is trying to accomplish. The following are definitions of the facing page information:

Most diagnostic tables have one result. This means that the conditions that existed when the DTC set are no longer present. The problem is not fixed, but the problem occurs intermittently. The problem was present but is not present now. The problem still needs to be diagnosed and repaired, if possible, because the concern is likely to return in the future. To diagnose a problem that is not currently present, gather information from the time when the DTC was set. You can accomplish this through the Snapshot data, the Freeze Frame data, and the driver's observations.

You can use the Snapshot data to determine if the data parameters fall within the normal operating range. For example, a Snapshot TP sensor parameter value of 100 percent taken while the engine was idling is not normal. This reading would indicate that the excessive TP sensor voltage resulted from an open in the sensor return circuit or a short to voltage on the sensor signal circuit. The driver's observations may give additional information about the system or area of the car that should be checked for an intermittent. For example, if an intermittent DTC P0300 is present, and the driver reports that the MIL only comes on in the rain, you would check the secondary ignition wiring, because it is likely that an ignition component is arcing to ground when wet.

This kind of information can be very helpful in diagnosing intermittent conditions. Sometimes, this may be the only information available about a fault. This information may provide the only chance to find the cause of an intermittent.

Many diagnostic tables will instruct you to check the terminal contact before replacing a component. This is done because the checks that are performed in the diagnostic tables only check the continuity of a circuit across a wire or in-line connection. Checking the terminal contact will prevent the replacement of good components, prevent comebacks due to intermittent connection problems, and make some repair jobs easier (e.g. replacing a terminal instead of a component). Check the terminal contact when you are instructed to do so.

Checking the terminal contact is easy as long as you have a supply of new terminals. Use the J-38125 terminal repair kit when checking the terminal contact. The kit contains a supply of all the terminals currently in use. Verify that the male terminals are straight and aligned with the other terminals in the row. The female terminals should be also be inspected for alignment and damage. Check the connections by taking a new male terminal of the same series (e.g. Metripack 150, Weatherpack, etc.) and connecting the male terminal to the female terminals being checked. The terminal should not fall out or be easily jarred out of the connection. The terminal should require some force for a disconnection. The force required to disconnect the terminals will vary with the size of the terminal. Larger terminals, such as those in the Metripack 630 series, should be very difficult to remove by hand. Smaller terminals, such as those in the Micropack series, should be easier to disconnect by hand but still should not fall out. Replace any damaged terminals. Refer to Wiring Repairs in Wiring Systems or the J-38125 terminal repair kit for identification of the terminal series and the repair procedures.