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. Be sure to read the On-Vehicle Service procedures for those components.

During the diagnosis or if a wiring problem is present, refer to Electrical Diagnosis in Engine Electrical . Electrical Diagnosis in Engine Electrical 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 sets. If the DTC does not re-set 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 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. It is critical that the technician understands what the customer's complaint is. Failure to understand this may lead to misdiagnosis or unnecessary diagnosis. Among other things, 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. Trying to diagnose a complaint 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 the problem. The diagnostic chart is a step by step procedure in order to determine the circuit or component that is the source of the problem. Wiring diagrams, text describing the system, test conditions, failure 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 (Battery disconnect, using a scan tool or 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 failure which may affect only one circuit, following the diagnostic procedures in this section will determine which circuit has a problem and where the problem is located.

If a diagnostic table indicates that the ECM connections or the ECM is the cause of a problem and the ECM is replaced, but does not correct the problem, one of the following conditions may be the reason:

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 (refer to Capture Info.). Once you have corrected and verified the problem, it is a good idea to 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 a repair once the 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 good that the problem is fixed.

Selecting this option on the scan tool allows the technician to record the Freeze Frame data. The scan tool stores the data. When starting the diagnosis, capture the Freeze Frame data. Later this information can be reviewed, even if the ECM or the battery has been disconnected, or if the DTCs were cleared during a diagnostic procedure.

The ECM snapshot feature is designed to assist in a diagnosis of intermittent problems. When an ECM snapshot is taken, a predetermined subset of the ECM's data parameters at that moment in time are stored for retrieval and 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 very useful in diagnosing intermittent engine problems. You may retrieve and use the stored data in order to determine if a particular input was incorrect when the problem occurred. Many of the ECM DTC trouble charts will use the Freeze Frame information in order to pinpoint a problem if the problem is not current. For example. a vehicle comes in with an intermittent rough engine condition and a scan tool Snapshot was taken while the engine was running rough. Several data parameters may be retrieved and an abnormal parameter be noted. The MAP sensor. the ECT sensor. the IAT sensor. and the Knock Sensor parameters may be compared to the normal values of those parameters and any values that are out of the normal range investigated. Also. the fuel injector pulse width or the spark advance may be retrieved. This may indicate a fueling or an ignition irregularity 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.

One of the most important checks 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 use of the charts. Check the following areas:

When diagnosing the Catera engine, you will almost certainly need to use the diagnostic procedures in this section or other engine sections. These diagnostic procedures are mostly 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 any test descriptions which 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 which reads Fault Not Present. This means that the conditions that were present when the DTC set are no longer there. This does not mean that the problem is fixed, this simply means that the problem is intermittent. The problem was present but is not present now. And the problem is likely to return in the future so the problem should be diagnosed and repaired if at all possible. The only way to diagnose a problem that is not present is to gather information from the time when the DTC was set. You can accomplish this process in two ways: through the Snapshot data, the Freeze Frame data, and the driver observations.

You can use the snapshot data to check the data parameters in order to determine if the data parameters fall within the normal operating range. For example, a snapshot TP sensor parameter value of 100 percent while the engine was idling is not normal and would indicate that the TP sensor voltage went high either due to an open in the sensor return circuit or a short to voltage on the sensor signal circuit. Driver observations may give additional information about the system or area of the car that should be checked for an intermittent. For example, while diagnosing an intermittent DTC P0300, you will find out that the driver noticed that the MIL only comes on in the rain. This would lead you to check the secondary ignition wiring, because it is likely that an ignition component is arcing to ground when wet.

This kind of information, while not airtight. may be very helpful in diagnosing intermittent conditions. Besides, this information may be the only information you can obtain about a failure and the only chance to find the cause of an intermittent.

Many diagnostic tables will have you checking the terminal contact before replacing a component. This is done because the checks that are performed in the diagnostic tables can only check the continuity of a circuit across a wire or in-line connection, not the continuity across the connection at a component. 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). For this reason, it is very important to 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 handy. The Terminal Repair Kit. J 38125, is a good source of terminals for checking the terminal contact. The kit contains a supply of all currently used terminals. In order to check the terminal contact. start by inspecting the male terminals. The male terminals should be straight and aligned with the other terminals in the row. These terminals should not be twisted. bent or otherwise damaged. The female terminal should be also be inspected for alignment and damage. Finally, take a new male terminal of the same series (e.g. Metripack 150, Weatherpack, etc,) and connect the male terminal to the female terminal(s) that are to be checked. The terminal should not fall out or be easily jarred out of connection. The terminal should require some force for a disconnection. The force that is required to disconnect the terminals will depend on the size of the terminal being checked. The larger terminals. the Metripack 630 series for example. should be very difficult to remove by hand. The smaller terminals, the Micropack series for example, should be easier to disconnect by hand but still should not fall out. Replace. do not repair. any damaged terminals. Refer to Electrical Diagnosis in Engine Electrical and the Terminal Repair Kit. J 38125. for identification of the terminal series and the repair procedures.