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 & 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.

Flashing MIL

Important: Any non-catalyst damaging misfire is a Type B DTC.

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 (10 psi). 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.

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:

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 through 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.

The following is an overview of DTC types and how information is stored in memory.

EMISSION RELATED

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, DTC becomes active.

Second ignition cycle with same fault - Failure Record 1 updates, 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.

FREEZE FRAME 0 will store the following:

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 10 different combinations of additional information ranging from 3 to 14 parameters for all of the powertrain DTCs.

The failure records for PCM will record all of this information plus additional information specific to the particular DTC. The failure record will be erased from the 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.

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.

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 display can be observed while driving the vehicle under the condition when the malfunction indicator lamp (MIL) or 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 intermittent operation.

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 problems.

After the 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:

1. Are the on-vehicle diagnostics working? This is determined by performing the vehicle on-board diagnostic (OBD) system check. Since this is the starting point for the diagnostic procedures or finding the cause of an emissions test failure, always begin here.
2. Is there a diagnostic trouble code stored? If a diagnostic trouble code is stored, go directly to the numbered diagnostic trouble code chart. This will determine if the fault is still present. If no diagnostic trouble code is stored, proceed to Step 3.
3. Scan serial data transmitted by the PCM. This involves reading the 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.

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:

1. Start the vehicle and run the engine until normal operating temperature is obtained.
2. Drive the vehicle at part throttle, with moderate acceleration and idle conditions until normal performance returns.
3. Park the vehicle and engage the parking brake with engine running.
4. On vehicles equipped with an automatic transaxle, place the transaxle in Drive.
5. On vehicles equipped with a manual transaxle, place the transaxle in Neutral.
6. Allow the vehicle to idle for about two minutes, until engine idle stabilizes. Make sure engine is at operating temperature.

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 oxygen sensor 1 voltage is varying, 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.

I/M ready status means a flag for each emission test has been set in the PCM. I/M ready status indicates that the vehicles 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.