Visual/Physical Underhood Inspection
A careful visual and physical underhood inspection must be performed as part of any diagnostic procedure or in finding the cause of a failure. This can often lead to fixing a problem without further steps. Inspect all vacuum hoses for correct routing, pinches, cuts or disconnects. Inspect all wires in the engine compartment for proper connections, burned or chafed spots, pinched wires, contact with sharp edges or the hot exhaust manifold. This visual/physical inspection is very important. It must be done carefully and thoroughly.
Knowledge and Tools Required
To use this manual most effectively, a general understanding of basic electrical circuits and circuit testing tools is required. One should be familiar with wiring diagrams, the meaning of voltage, ohms, amps, the basic theory of electricity, and understand what happens when a wire opens or shorts.
To perform system diagnosis, the use of a diagnostic scan tool is required. A 12-volt test light, digital volt ohmmeter with 10 megohms impedance, vacuum gage, and jumper wires are also required. Please become acquainted with the tools and their use before attempting to diagnose a vehicle.
Malfunction Indicator Lamp (MIL)
The malfunction indicator lamp (MIL) or SERVICE ENGINE SOON telltale is in the instrument cluster and it informs the driver that a problem has occurred and that the vehicle should be taken for service as soon as reasonably possible.
As a bulb and system check, the telltale will come ON with the key ON and the engine not running. When the engine is started, the telltale will turn OFF. If the telltale remains ON, the self-diagnostic system has detected a problem. If the problem goes away, the telltale will turn OFF in most cases after three trips, and a diagnostic trouble code will remain stored in the PCM.
If the telltale remains ON while the engine is running, or when a malfunction is suspected due to a driveability problem, an on-board diagnostic system check, OBD system check, must be performed. These checks will expose malfunctions which may not be detected if other diagnostics are performed prematurely. Refer to the "Vehicle On Board Diagnostic (OBD) System Check" in the "Data Link Communications" section in the Body/Electrical Volume II service manual.
Service Telltale
The SERVICE telltale is in the instrument cluster and informs the driver that a problem has occurred requiring the vehicle to be taken in for service as soon as reasonably possible. This telltale is used for non-emission related failures, which without being serviced, could lead to subsystem component damage. As a bulb check, the telltale will come ON for 2-3 seconds and then turn OFF. If the telltale remains ON, the self-diagnostics system has detected a fault. If the problem corrects itself or goes away, the telltale will turn OFF three seconds after the PCM diagnostic test passes. A diagnostic trouble code will be stored in Freeze Frame/Failure Records.
Diagnostic Information
The diagnostic trouble tree charts and functional checks in this manual are designed to locate a faulty circuit or component through logic, based on the process of elimination. The charts are prepared with the requirement that the vehicle functioned correctly at the time of assembly and that there are no multiple failures. Trouble tree charts are designed to be used only when a diagnostic trouble code is active.
The PCM performs self-diagnostics of powertrain control functions. The PCM communicates faults with a system of diagnostic trouble codes (DTCs). These DTCs may or may not turn ON the MIL (SERVICE ENGINE SOON telltale) or SERVICE telltale to alert the driver.
Intermittent Malfunction
An intermittent problem may or may not set a diagnostic trouble code. If it is an intermittent problem, a diagnostic trouble code chart is not used. Consult the Diagnostic Aids on the page facing the diagnostic chart corresponding to the intermittent diagnostic trouble code. Also, refer to symptom pages in the front of the manual. A physical/visual inspection of the applicable sub-system most often will resolve the problem. Use of the customer snap-shot and scan tool snap-shot may isolate the malfunction. Refer to Symptom Diagnosis section for customer snap shot and Service Stall System (SSS) user guide for scan tool snap-shot operation.
DLC Scan Tool
The PCM can communicate a variety of information through the DLC. This data is transmitted at a high-frequency which requires a scan tool for interpretation. There are several scan tools available for reading this information.
With an understanding of the data which the tool displays and knowledge of the circuits involved, the tool can be very useful in obtaining information which would be more difficult or impossible to obtain with other equipment.
Scan tools do not make the use of diagnostic charts unnecessary, nor can they indicate exactly where a problem is in a particular circuit. Trouble code diagnostic charts incorporate diagnosis procedures using a scan tool where possible and most charts require the use of a scan tool when it is applicable.
The scan tool has the ability to save time in diagnosis and prevent the replacement of good parts. The key to using the scan tool successfully for diagnosis lies in the technician's ability to understand the system being diagnosed, as well as an understanding of the operation and limitations of the scan tool. The technician should read the tool manufacturer's operating manual to become familiar with the tool operation.
Reading Diagnostic Trouble Codes
The provision for communicating with the PCM is the data link connector (DLC) which is located under the instrument panel. It is used to retrieve DTCs, failure record information, live data parameters and PCM reprogramming.
The diagnostic trouble codes stored in PCM memory can be read only through a hand-held diagnostic scan tool plugged in the DLC connector when the ignition is turned ON.
Clearing Diagnostic Trouble Codes
Clearing diagnostic trouble codes is accomplished by choosing the "Clearing Trouble Codes" option in the scan tool DTC menu. Disconnecting the battery for 60 seconds, and in some cases longer, will clear DTCs, active and history from PCM memory. Failure Record information will also be cleared.
Diagnostic Trouble Code (DTC) Types
Government regulations based on SAE standards, have mandated the automotive industry to adopt a common numbering system for emission diagnostic trouble codes (DTCs). The numbering system requires one alpha followed by four numeric numbers to make up the code (Pxxxx). The "P" indicates a powertrain diagnostic trouble code. This means that a DTC number for a throttle position sensor will be the same for Saturn, as for any other manufacturer. (P0 before the code means industry standard and P1 before the code means manufacturer specific.)
Along with the change in the numbering system is the ability to know if a diagnostic test has been performed on each system. This means that it can be determined if a test has been run on a subsystem and whether the result was passed, failed, or pass and fail.
Since the MIL (SERVICE ENGINE SOON telltale) is restricted to being illuminated only for emission-related faults, there have been four code types assigned. These types will be referenced to as type A, B, C and D.
Type A DTCs: are emission-related faults that will turn ON the MIL at the first occurrence of a fail condition.Type B DTCs: are emission-related faults that turn ON the MIL if a fault occurs in two consecutive ignition cycles.
Type C DTCs: are non-emission faults that may cause the SERVICE telltale to be illuminated. If a test passes after a fault is detected, the SERVICE telltale will turn OFF after about 3 seconds, but the fault remains stored in history and a failure record is stored.
Type D DTCs: are used to detect non-emission faults, and aid in diagnosis, but do not turn ON any telltales.
EMISSION RELATED
Important: Any non-catalyst damaging misfire is a type B DTC.
TYPE A - First fault during first ignition cycle - Freeze Frame "0" stores DTC plus information, Failure Record 1 stores (same record as freeze frame "0") and updates with every failed test, DTC becomes active, MIL illuminates.
TYPE B - First fault during first ignition cycle - Freeze Frame "0" stores DTC plus information, Failure Record 1 stores (same record as freeze frame "0") and updates with every failed test.
Second ignition cycle with same fault - Failure Record 1 updates, DTC becomes active, MIL illuminates.
NON-EMISSION RELATED
TYPE C - First fault during first ignition cycle - Failure Record 1 stores DTC plus information and updates with every failed test, DTC becomes active, SERVICE telltale illuminates.
TYPE D - First fault during first ignition cycle - Failure Record 1 stores DTC plus information and updates with every failed test, DTC becomes active.
It is important to remember that the DTC as well as any stored failure record information will be erased if the vehicle battery has been disconnected or a clear DTC command is issued to the PCM. DTCs and failure record information will also be erased if 40 passes (only one pass per ignition cycle is counted) have been recorded without any faults. All emission (DTC type A or B) and non-emission (DTC type C and D) codes require this 40 pass criteria.
Flashing MIL
The MIL (SERVICE ENGINE SOON telltale) will flash if a catalyst damaging misfire occurs with vehicle speed above 50 km/h (30 mph) and MAP greater than 70 kPa. Since the catalyst damaging misfire is a type B DTC, the MIL will turn OFF if the misfire terminates in that ignition cycle. However, if a catalyst damaging misfire occurs on the second consecutive trip and terminates in that ignition cycle, the MIL will again flash, but will be illuminated ON steady as soon as the catalyst damaging misfire no longer exists. NOTE: Any non-catalyst damaging misfire is a type B DTC.
DTC Test Description
The self-diagnostic tests run to determine if a subsystem has passed or failed a specified test. After a test is performed, the PCM will record test PASS/FAIL criteria for display with a scan tool. Each DTC will have three records and are as follows:
| • | LAST TEST - Indicates whether the diagnostic reported a test passed or failed the last time it ran, even if it was not run this ignition cycle. |
| Possible Display: PASSED, FAILED, NOT RAN |
| • | THIS IGNITION - Indicates if the diagnostic test has run this ignition cycle and the results of that test. |
| Possible Display: PASSED, FAILED, PASS AND FAIL, NOT RAN |
| • | SINCE CLEAR - Indicates if diagnostic test has run since DTCs were cleared and the results of that test. |
| Possible Display: PASSED, FAILED, PASS AND FAIL, NOT RAN, HISTORY |
Freeze Frame/Failure Records
Important: A "fault" mentioned below is an error flag set in the PCM indicating a diagnostic test has run and failed. The "fault" is designated by a DTC in Freeze Frame/Failure Record information, but does not suggest that the DTC is active.
The scan tool displays vehicle malfunction history information located in the Diagnostic Trouble Code menu under the subheading Freeze Frame/Failure Records. Freeze Frame/Failure Records contain a maximum of 7 records, 1 FREEZE FRAME plus 6 FAILURE RECORDS. Government regulations mandate that all emission related faults store freeze frame data, designated as Failure Record "0" on scan tool, at time of the first failed test. The first emission related fault that sets after a code clear or battery disconnect is stored in freeze frame and can only be overwritten by Fuel Trim and Misfire faults. All other emission related faults that set, will not be stored in freeze frame unless the initial fault is cleared. Failure Records 1- 6 on scan tool store information at time of first fault and are updated every time the PCM runs a test and fails the diagnostic. If another fault were to occur, the DTC in record 1 would be moved to record 2 and the new DTC would be placed in record 1 (first in, first out). If a new failure is detected while all failure records are full, the number 6 record will be erased (oldest) and the newest fault will be updated to failure record 1.FREEZE FRAME 0 will store the following: | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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Each DTC will store all of the above parameters in the Failure Record as well as additional information based on the specific DTC. There are 7 different combinations of additional information ranging from 3-33 parameters for all of the powertrain DTCs.
The failure records for the PCM will record all of the above information plus the additional information specific to the particular DTC. The failure record will be erased from PCM memory during a code clear, battery disconnect or after 40 passes, only one pass per ignition cycle is counted, without any faults detected in that circuit.
Trip
A "TRIP" is defined as a key ON, Run, key OFF cycle in which all the criteria were met for a given diagnostic test to run. During any given drive cycle, key ON, Run, key OFF criteria may or may not have been met to perform all diagnostic tests. No "TRIP" will occur for a given diagnostic until the vehicle is driven in such a manner as to meet all the enable criteria. If a test is desired on a particular subsystem to determine if it "passes" or "fails," refer to the specific DTC in this manual for test parameters. The parameters are calibrated conditions the PCM requires in order to perform the test. If a test has not been run, monitor the scan tool PASS/FAIL DTC status at the time the parameters have been met.
Warm Up Cycle
A warm-up cycle is defined as engine temperature reaching a minimum of 71°C (160°F) and a rise in temperature of at least 22°C (72°F) over the course of a trip.
Scan Tool Usage With Intermittents
In some scan tool applications, the data update rate makes the tool less effective than a voltmeter, such as when trying to detect an intermittent problem which lasts for a very short time. However, the scan tool does allow one to manipulate wiring harnesses or components under the hood with the engine not running, while observing the readout of the scan tool.
Scan tool data display can be observed while driving the vehicle under the condition when the malfunction indicator lamp (MIL) or the SERVICE telltale turns ON momentarily or when engine driveability is momentarily poor. An assistant should be used when driving a vehicle and using a scan tool. If the problem seems to be related to certain parameters that can be checked on the scan tool, they should be checked while driving the vehicle. If there does not seem to be any correlation between the problem and any specific circuit, the scan tool can be checked at different conditions, watching for a period of time to see if there is any change in the readings that would indicate an intermittent problem.
The scan tool is also an easy way to compare the operating parameters of a poorly operating engine with those of a known good one. For example, a sensor may shift in value, but not set a diagnostic trouble code. Comparing sensor readings with those of a known good vehicle may uncover the problem.
The PCM has the ability to take a snapshot when a diagnostic trouble code is stored. This information can be found with a scan tool listed under Failure Record information. This information can be very useful when attempting to troubleshoot intermittent diagnostic trouble codes.
The "customer snap-shot" feature can also be used for intermittents. Refer to DTC P1624 for explanation.
Vehicle On-Board Diagnostic (OBD) System Check
After a visual/physical underhood inspection, the vehicle on-board diagnostic (OBD) system check is the starting point for all diagnostic procedures or finding the cause of an emissions test failure.
The correct procedure to diagnose a problem is to follow three basic steps:
- Are the on-vehicle diagnostics working? This is determined by performing the Vehicle On-Board Diagnostic (OBD) System Check located in the "Data Link Communications" section in the Body/Electrical Volume II service manual. Since this is the starting point for the diagnostic procedures or finding the cause of an emissions test failure, always begin here.
- Is there a diagnostic trouble code stored? If a diagnostic trouble code is stored, go directly to the numbered diagnostic trouble code chart. The chart will identify if the fault is still present. If no diagnostic trouble code is stored, proceed to Step 3.
- Scan serial data transmitted by the PCM. This involves reading information available on the serial data stream with a scan tool. The meaning of the various displays can be found in the scan tool data definition section. Expected readings under a particular operating condition can be found in the scan tool data definition section.
PCM Learning Ability
The PCM has a learning ability that allows it to make corrections for minor variations in the fuel system to improve driveability and engine idle. If the battery is disconnected, or the PCM is replaced, the PCM must go through a learning process. Until the PCM has gone through this learning process, a change may be noted in the driveability, idle or shift feel (Automatic Transaxle only) of the vehicle. To allow the PCM to relearn, the following steps must be performed:
| • | Start the vehicle and run the engine until normal operating temperature is obtained. |
| • | Drive the vehicle at part throttle, with moderate acceleration and idle conditions until normal performance returns. |
| • | Park the vehicle and engage the parking brake with engine running. |
| • | On vehicles equipped with an automatic transaxle, place the transaxle in Drive. |
| On vehicles equipped with a manual transaxle, place the transaxle in Neutral. |
| • | Allow the vehicle to idle for about two minutes, until engine idle stabilizes. |
Open and Closed Loop Operation
The PCM operates in two different fuel control modes: Open Loop and Closed Loop. Whenever the vehicle is first started, the PCM operates in open loop fuel control. When the PCM determines that the engine oxygen sensor 1 voltage is greater than 600 mV or less than 300 mV, it will go into closed loop fuel control operation. Closed loop fuel control operation will be maintained as long as certain parameters are met.
In closed loop fuel control, the PCM varies the fuel to the engine according to signals received from the oxygen sensor 1, located in the exhaust manifold. The oxygen sensor 1 varies a voltage signal to the PCM indicating the oxygen content of exhaust gases. If the oxygen sensor 1 signals that the air/fuel mixture is lean, the PCM will increase the amount of fuel to the engine. If the oxygen sensor 1 signals that the air/fuel mixture is rich, the PCM will decrease the amount of fuel to the engine. During closed loop fuel control operation, the PCM is constantly adjusting the amount of fuel to the engine according to signals received from the oxygen sensor 1, in order to obtain a 14.7:1 air fuel ratio.
If there are any oxygen sensor 1 related diagnostic trouble codes or if the oxygen sensor 1 does not switch between rich and lean, less than 300 mV to greater than 600 mV, the PCM will not go into closed loop fuel control operation.
Inspection/Maintenance (I/M) Ready Status
I/M ready status means a flag for each emission test has been set in the PCM. I/M ready status indicates that the vehicle on-board emissions diagnostics have been run. The status flag does not display passed or failed, only that the on-board diagnostic test was completed. The I/M ready status is used for tests that are only run once per trip.
