OBD-II System

GENERAL
The 1995 Legacy is equipped with "On Board Diagnosis TWO". This svstem combines engine and automatic transmission "Diagnostic Trouble Codes", (DTC). This is done because of the impact that these drive train svstems have on the creation and control of vehicle emissions.

There will be a total of 63 DTC's on automatic transmission equipped vehicles, less for manual transmission equipped vehicles. The DTCs can be read with a "Generic Scan Tool" or the "Select Monitor". The Select Monitor's functions have been enhanced to more efficiently assist with the diagnostics on the "OBD-II" system.

The on-board diagnosis system detects and indicates a fault in various inputs and outputs of the complex electronic control. The malfunction indicator lamp (CHECK ENGINE light) on the instrument panel indicates occurrence of a fault or trouble.
When the malfunction indicator lamp comes on by the ECM having diagnosed occurrence of faults, the diagnostic trouble code (DTC) and the freeze frame engine condition are stored in the ECM.
On the OBD-II conformable car, it is necessary to connect the Subaru Select Monitor (SSM) or General Scan Tool (GST) to data link connector in order to check the DTC.
The SSM and GST not only can read out the DTC but also can erase the DTC or read out the freeze frame data and other pieces of engine data.
Further, against such a failure or sensors as may disable the drive, the fail-safe function is provided to ensure the minimal driveability.

The number of "F" modes have increased to 56. Seven of these modes are used to retrieve "Freeze Frame Data", which is set into the ECM memory when a DTC is set. This function of the ECM gives an advantage over having trouble code memory only. Information included in the Freeze Frame functions are:
- Load Data
- Engine Coolant Temperature
- Throttle Position Signal
- Long Term Trim
- Intake Nianifold Pressure
- Engine Speed
- Vehicle Speed

Accessing of this information is accomplished by scrolling or entering the correct "F" mode. The Sociery of Automotive Engineers in cooperation with the EPA have in accordance with regulation J2012 of the 1990 Clean Air Act, established DTC's that are to be used by the automobile industry beginning with the 1995 model year. (Carry over models may have a waiver that excludes them from the J2012 mandate.) DTC's that use a "P0" prefix are SAE assigned. DTCs that use a "P1" prefix are categorized by SAE but are assigned bv a vehicle manufacturer.

OBD-II checks a component and its operation similar to OBD-I, which is used on current vehicle models. OBD-I for example, checks the "Throttle Position Sensor" by monitoring for minimum and maximum voltage signals. OBD-II also performs this function.
For example, examine DTC P0120.
When either condition a) or b) persists
a) Sensor output is less than 0.1 volts
b) Sensor output is greater than 4.9 volts
Condition a or b will cause the DTC to register in the ECM memory on OBD-I or OBD-II systems.

In addition to the above, OBD-II will monitor for performance of the Throttle Position Sensor. For example, examine DTC P0121.
When condition a) or b) persists after warm
a) Although engine speed is greater than 2500 rpm, throttle position sensor output is less than 0.75 volts.
b) Although engine speed is equal to or greater than 600 rpm, or equal to or less than 900 rpm, throttle position sensor output is greater than 1.5 volts.

The 1995 Legacy uses an inspection mode to check for current malfunctioning OBD-II circuits or components. Inspection mode is accessed by connecting the green, two pole, single wire inspection mode connectors, located under the driver's dash area. The service manual instructions must be followed or incorrect results will be achieved. Inspection mode will not check OBD-II performance tests, however it will display a DTC on the select monitor which may have resulted from a performance test during normal vehicle operation. During normal vehicle operation the inspection mode connector is disconnected. The FB1 mode of the select monitor displays the trouble code that caused the check engine light to illuminate, however in many cases two failures of a circuit or a component are necessary to generate a trouble code and turn on the check engine light. If one failure exists of a DTC, the inspection mode FB0 will display it while the OBD-II mode FB1 will not.

Performance tests require very specific vehicle operating conditions. For example, one of the performance tests the computer conducts is on the EGR system. The following conditions must exist for the test to be conducted.
- Throttle Angle 5.76 - 24.96°
- Engine Speed 1800 - 2800 rpm
- Coolant Temperature Greater than 70°
- Barometric Pressure 5O7 mmHg

Once all of the operating conditions are met, the computer will turn on the EGR solenoid and measure the intake manifold pressure for approximately 1 second. The pressure is memorized and the EGR off pressure is checked and memorized. The computer will then turn the solenoid back on for 1 second and recheck the intake manifold pressure. The results must be more than 6mm hg and less than 42mm hg to be judged working properly.

The DTC will set and the check engine light will illuminate if a fault is registered during two consecutive trips. A trip is defined as the conditions in which performance testing of emission control and on-board diagnostics systems are evaluated. Trip parameters will vary according to the system being tested.
For DTC's that require two failed trips to set, the second consecutive trip must also result in testing outside pre-established parameters. If the performance test during the second trip is judged to be operating within pre-established parameters, the first trip failure will be erased.

FAIL-SAFE FUNCTION
For the part which has been judged faulty in the on-board diagnosis, the ECM generates the associated pseudo signal (only when convertible to electric signal) and carries out the computational processing. In this fashion, the fail-safe function is performed.