Subaru OBD2 Decoding Explained

Subaru OBD2 Decoding

Decoding the Codes

One of the main reasons that emissions codes have been confusing is because, as emission systems evolved over the last dozen years, more codes have been added to cover newer and more complex devices.

Subaru OBD2 diagnostic trouble codes (DTCs) are identified by a standardized system. Each letter or digit in the 5-place system represents a factor that tells a portion of the total story.

Let’s look at an example. The DTC P0442 can be decoded using the following system:

The first place “P” in the P0442 code tells in which subsystem the fault is occurring:

P = Powertrain
B = Body
C = Chassis
U = Network Communication

Now, we know that the problem is in the powertrain.

The second place “0” in the P0442 code tells whether the DTC is a generic Subaru OBD2 fault as standardized by the Society of Automotive Engineers (SAE) or one developed my the vehicle manufacturer:

0 = SAE
1 = Manufacturer

OK, it’s a standard Subaru OBD2 DTC.

The third place “4” in the
P0442 code indicates the affected

1 = Air/Fuel Control
2 = Fuel System (Injectors)
3 = Ignition System/Misfire
4 = Auxiliary Emission Controls
5 = Vehicle Speed/Idle Control
and Auxiliary Inputs
6 = Computer System (PCM or
7 = Transaxle/Transmission
8 = Transaxle/Transmission

The problem lies in the emission control systems, but further information will be required to pinpoint the problem.

The fourth and fifth places “42” in the P0442 code will give the specific location of the faulty circuit or component. These digits vary by subsystem, but referring to DTC charts or reading with a quality scan tool will identify the exact problem. In this case, the “42” indicates: Evaporative Emission Control System Malfunction (Small Leak – 1 mm/ 0.04 in).

In many 2001 and prior models, a P0442 DTC may be caused by a loose gas cap. In 2002, the DTC P0457 was added to differentiate between an EVAP system leak and a loose gas cap.

Further Reading: