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Subaru OBD2 Decoding Explained

Subaru OBD2 Decoding

Few types of diagnostic trouble codes can be more confusing than those dealing with emission problems. From the beginning of mandatory Subaru OBD2 in 1996, more codes have been added and some have changed. Here’s a look at how Subaru of America, Inc. has added and streamlined P0400-series DTCs.

Subaru OBD2 Decoding: Locations of the OBD2 ports in various Subaru cars.
Subaru OBD2 Decoding: Locations of the OBD2 ports in various Subaru cars.

Emissions-related Subaru OBD2 diagnostic trouble codes (DTCs) have evolved over the last dozen years to more precisely pinpoint the problems in automotive systems. The handful of emissions codes used for On-Board Diagnostic (OBD) systems on the late 1980s and early 1990s has grown to nearly a hundred today. Over that time, many DTCS have been modified to more accurately reflect the cause, while others have been added to the list to address issues with advancing technology.

In order to understand how these factors affect Subaru OBD2 vehicles, it’s necessary to first look at the history of emissions control, on-board diagnostics and the DTC coding system.

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AWD, Drivetrain, forester, gc8, Impreza, Legacy, STi, Subaru, Suspension, WRX

Wheel Bearing Guide Subaru

Wheel Bearing Subaru Guide:

The amount of force exerted on wheel bearings is astounding. Each bearing is required to smoothly control the rotation of the wheel to the tune of about a thousand revolutions per mile, support the transfer of power to the wheels for rapid starts and sudden stops, and handle the powerful lateral twisting force of the tires changing direction against the pavement — all while supporting a vertical load of hundreds of pounds. And, we expect them to perform flawlessly just about forever? Not realistic.

Wheel Bearing Guide Subaru: This tapered roller bearing was damaged by faulty seals that allowed water and dirt to enter the bearing.
Wheel Bearing Guide Subaru: This tapered roller bearing was damaged by faulty seals that allowed water and dirt to enter the bearing.

The “Achilles Heel” of a wheel bearing is the seal. Although wheel bearings can fail due to damage, improper installation or material imperfection, the most common cause of failure is the seal losing its ability to hold the lubricating grease in and/or dirt and water out.

However, the best seal, applied to the best wheel bearing, cannot be expected to last if not correctly installed. This primer can help you properly service Subaru wheel bearings.

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2.5RS, AWD, Drivetrain, EJ25, forester, Impreza, Legacy, Old School Subaru, Subaru, Transmission, Turbo, USDM, WRX

Clutch Pedal Sticking Subaru

Clutch Pedal Sticking Subaru:

If you encounter a clutch pedal not returning completely after being engaged, or if there is a spongy or light clutch pedal feel while shifting, the following repair method should be followed.

Clutch Pedal Sticking Subaru: This condition may affect certain manual transmission vehicles with a hydraulic clutch system under certain weather conditions.
Clutch Pedal Sticking Subaru: This condition may affect certain manual transmission vehicles with a hydraulic clutch system under certain weather conditions.

This condition may affect certain manual transmission vehicles with a hydraulic clutch system under certain weather conditions.The affected manual transmission Subaru models are as follows:

1995-2002 Legacy
1997-2007 2.5L Impreza
1998-2003 Forester

To correct this condition you must replace the parts in the chart that match your vehicle using the following procedures:

Replacement Clutch Parts:

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2.5RS, AWD, Drivetrain, forester, gc8, Impreza, Legacy, STi, Subaru, TMIC, Turbo, USDM, WRX

Subaru Cold Weather And Driveability

Subaru Cold Weather And Driveability:

The Winter season brings cold weather to many parts of the country, and with it the traditional driveability problems.

Subaru Cold Weather And Driveability: The Winter season brings cold weather to many parts of the country, and with it the traditional driveability problems.
Subaru Cold Weather And Driveability: The Winter season brings cold weather to many parts of the country, and with it the traditional driveability problems.

Before you push the panic button on Subaru cold weather and driveability problems, remember:

• No vehicle runs as well when it is cold as it does when it is at normal operating temperature.

• You have been operating the vehicle in more moderate temperatures and has gotten accustomed to the way it has been running. Now it is colder and things are not working the same.

• Some areas of the country may be using gasoline blended for warmer temperatures. These fuels normally do not atomize as well in cooler temperatures.

• Oxygenated and reformulated fuels that are in use in many parts of the country are normally harder to ignite in cold cylinders.

• Many drivers get their gas at one station because it may be close to home or work. Question them about this and if this is true, suggest they try a different brand of gas. It may take a couple tanks before any improvement is noticed. Different manufacturers blend their fuels differently.

• The 4EAT has a temperature sensor in the ATF and the Transmission Control Unit (TCU) will not allow an up-shift into 4th gear until the ATF has reached a specific temperature. This 4EAT design characteristic may be interpreted as a driveability problem by a driver who is not familiar with 4EAT operation.

There are many reasons for Subaru cold weather and driveability issues during cooler weather. Spending a few minutes with your Subaru and look over the points listed above should eliminate misconceptions about the Subaru cold weather performance and driveability characteristics of Subaru vehicles.

 

2.5RS, Drivetrain, Ej20, ej20G, EJ25, EJ257, forester, gc8, Impreza, Legacy, Old School Subaru, STi, Subaru, USDM, WRX

Battery Charging Subaru:

Battery Charging Subaru:

Batteries low in voltage (below 11.6 volts) need to be specially charged. A battery at this voltage is heavily sulfated and needs either a very long, slow charge, or a very high initial charge voltage.

Battery Charging Subaru: Batteries low in voltage (below 11.6 volts) need to be specially charged. A battery at this voltage is heavily sulfated and needs either a very long, slow charge, or a very high initial charge voltage.
Battery Charging Subaru: Batteries low in voltage (below 11.6 volts) need to be specially charged. A battery at this voltage is heavily sulfated and needs either a very long, slow charge, or a very high initial charge voltage.

The battery should be left on the battery charger for at least two days. Since the acid in the battery will mostly be stratified, it needs sufficient overcharge to mix. Even after a two day charge, the battery still may only come to 60-80 percent of capacity and may need to be cycled to come to full charge. If possible, once the battery is fully charged by this method, it’s advisable to finish with a constant 1 amp for an additional 24 hours.

A battery that is below 11.6 volts can also be hydrated. This means there is lead sulfate in the separator that will form lead shorts once the battery charges. Because of these shorts, the battery may self discharge once the battery has been recharged.

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2.5RS, AWD, Drivetrain, Ej20, ej20G, EJ25, EJ257, exhaust, forester, gc8, Impreza, Legacy, Old School Subaru, STi, Subaru, WRX

Emission Testing State Subaru

 Emission Testing Subaru:

Emission testing of a Full-Time 4WD or all-wheel-drive vehicle must never be performed on a single two-wheel dynamometer, nor should a state I/M program inspector or its contractors install the FWD fuse in the engine compartment. Attempting to do so will result in uncontrolled vehicle movement and may cause an accident or injuries to persons nearby.

State Emission Testing Subaru: Emission testing of a Full-Time 4WD or all-wheel-drive vehicle must never be performed on a single two-wheel dynamometer
State Emission Testing Subaru: Emission testing of a Full-Time 4WD or all-wheel-drive vehicle must never be performed on a single two-wheel dynamometer.

Resultant vehicle damage due to improper testing is not covered under the SUBARU Limited Warranty and is the responsibility of the state I/M Program or its contractors or licensees.

The 1990 Clean Air Act Amendments require the Environmental Protection Agency (EPA) to implement programs to reduce air pollution from motor vehicles. Certain states are required to adopt either a “basic” or “enhanced” vehicle Inspection/Maintenance (l/M) Program, depending on the severity of their air pollution problem.

The ‘enhanced’ I/M emission testing simulates actual driving conditions on a dynamometer and permits more accurate measurement of tailpipe emissions than the ‘basic’ I/M test, which measures emissions only during engine operating conditions at idle and 2500 RPM. The ‘enhanced’ l/M test also includes a pressure check to identify evaporative emissions leaks in the fuel system.

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2.5RS, Drivetrain, Ej20, ej20G, EJ25, EJ257, exhaust, forester, gc8, Impreza, Intake, Legacy, Old School Subaru, STi, Subaru, Tuning, WRX

Evaporative Emissions Testing Subaru

Evaporative Emissions Testing Subaru:

A major component of the Subaru OBD-II system is the system’s ability to monitor the evaporative emissions system. Today’s vehicles are producing very low emissions from the tailpipe, so it has become increasingly important to monitor and contain emissions from other vehicle sources.

Evaporative Emissions Testing Subaru: A major component of the Subaru OBD-II system is the system’s ability to monitor the evaporative emissions system.
Evaporative Emissions Testing Subaru: A major component of the Subaru OBD-II system is the system’s ability to monitor the evaporative emissions system.

A potentially large source of emissions is the vehicle’s fuel system. If not properly contained, vapors escaping from the fuel tank could produce a larger quantity of harmful emissions while the vehicle was standing still than what would be emitted via the tailpipe when the engine was running and the vehicle was driving down the road.

The Subaru OBD-II system monitors the evaporative emissions system by drawing the system to a negative pressure. If the system holds vacuum, it passes the test. If the system fails to hold vacuum for the prescribed period, it fails and a diagnostic trouble code (DTC) P04440 is stored in the ECM memory. The malfunction indicator light (MIL) also comes on in the dash to alert the driver to the problem.

The charts that follow were collected through the data link connector using the New Select Monitor (NSM), during the diagnosis of a DTC P0440 on a 1997 Subaru Legacy 2.5 liter. We’ll begin with a description of system operation under normal operating conditions.

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2.5RS, Drivetrain, Ej20, ej20G, EJ25, EJ257, forester, gc8, Impreza, Legacy, Old School Subaru, STi, Subaru, USDM, WRX

Knock Sensor diagnosis for Subaru

Knock Sensor diagnosis for Subaru:

This is a simple overview on diagnosing knock sensor issues with your Subaru Impreza/Forester/Legacy/Etc.

The knock sensor is designed to sense knocking signals from each cylinder. The knock sensor is a piezo-electric type element which converts knocking vibrations into electrical signals.
The knock sensor is designed to sense knocking signals from each cylinder. The knock sensor is a piezo-electric type element which converts knocking vibrations into electrical signals.

The knock sensor is designed to sense knocking signals from each cylinder. The knock sensor is a piezo-electric type element which converts knocking vibrations into electrical signals. The electrical signal is sent to the ECM, which changes the ignition timing to reduce the engine knock or ping. For this system to work correctly, the knock sensor must first hear the engine ping. The driver of the vehicle may also hear a small engine ping. A delay of approximately 1-2 seconds is normal, depending on the fuel quality, engine load, air temp, etc. At this time, the ECM will retard the timing.

This function can be viewed on the Select Monitor RTRD mode. When the knock is eliminated, the timing is gradually advanced to the specified setting. If engine ping is heard again this process is repeated. This will continue until the knock sensor no longer hears the engine knock or ping.

Note: This is a normal operation of the knock sensor. Do not try to repair it.

The next page will discuss asking the right questions on diagnosing knock sensor failures.

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