Subaru's will flash trouble codes on the LED light of the oxygen moniter of the ECU (Electronic Control Unit), with the key ON (engine off). Long pulses (1.2 seconds) indicate tens and the short pulses (.2 seconds) indicate ones.
See OBD II Codes from a code reader below.

88 and later models with Single-Point Fuel Injection
11 Crank angle sensor or circuit
12 Starter switch or circuit
13 Crank angle sensor or circuit
14 Fuel injector - abnormal output
21 Coolant temperature or circuit
23 Air flow meter or circuit
24 Air control valve or circuit
31 Throttle sensor or circuit
32 Oxygen sensor or circuit
33 Vehicle Speed Sensor or circuit
34 EGR solenoid or circuit
35 Purge control solenoid or circuit
42 Idle switch or circuit
45 Kick-down control relay or circuit
51 Neutral switch continuously in the on position
55 EGR temperature sensor or circuit
61 Parking switch or circuit
1988 and later models with Multi-Point Fuel Injection (including Legacy)
11 Crank angle sensor or circuit
12 Starter switch or circuit
13 Cam position sensor or circuit (TDC sensor on Justy)
14 Fuel injector no. 1 (Legacy, Impreza, Justy, SVX); Fuel injector 1 and 2 (XT, Loyale, GL, DL)
15 Fuel injector no. 2 (Legacy, Impreza, Justy, SVX); Fuel injector 3 and 4 (Loyale, GL, DL); Fuel injector 5 and 6 (XT6)
16 Fuel injector no. 3 (Legacy, Impreza, Justy, SVX); Fuel injector 3 and 4 (XT)
17 Fuel injector no. 4 (Legacy, Impreza, SVX); Fuel injector 1 and 2 (XT6)
18 Fuel injector no. 5 (SVX)
19 Fuel injector no. 6 (SVX)
21 Coolant temperature sensor or circuit
22 Knock sensor or circuit (right side on SVX)
23 Air flow meter or circuit (exc. Justy); Pressure sensor (Justy)
24 Air control valve or circuit (exc. Justy); Idle speed control solenoid valve (Justy)
25 Fuel Injector 3 and 4 (XT6)
26 Air temperature sensor (Justy)
28 Knock sensor no. 2 (SVX, left side)
29 Crank angle sensor (SVX, no. 2)
31 Throttle position sensor or circuit
32 Oxygen sensor or circuit (no. 1, right side on SVX)
33 Vehicle speed sensor (VSS) or circuit
34 EGR solenoid valve
35 Canister purge solenoid or circuit
36 Air suction solenoid valve (Impreza); Igniter circuit (Justy)
37 Oxygen sensor (no. 2, left side , SVX)
38 Engine torque control (SVX)
41 Air/fuel adaptive control
42 Idle switch or circuit
43 Throttle switch (Justy)
44 Wastegate duty solenoid (turbo)
45 Pressure duty solenoid (turbo)/atmospheric pressure sensor circuit (non-turbo)
49 Airflow sensor
51 Neutral switch (manual transmission)/inhibitor switch (automatic transmission)
52 Parking brake switch (exc. Justy)/Clutch switch (Justy)
55 EGR temperature sensor
56 EGR system
61 Parking brake switch (Loyale)/Fuel tank pressure control solenoid valve (Impreza)
62 Fuel temperature sensor (Impreza)/Electric load signal (Justy)
63 Fuel tank pressure sensor (Impreza)/Blower fan switch (Justy)
65 Vacuum pressure sensor

If you connect a OBD II Reader you will get the following codes.
See the DTC page for P0.. (generic) codes and Glossary for terms and acronyms..

Subaru OBD2 Codes
P1086 Tumble generator valve #2 (LH) position sensor circuit low input
P1087 Tumble generator valve #2 (LH) position sensor circuit high input
P1088 Tumble generator valve #1 (RH) position sensor circuit low input
P1089 Tumble generator valve #1 (RH) position sensor circuit high input
P1090 Tumble generator valve #1 (RH) malfunction (stuck open)
P1091 Tumble generator valve #1 (RH) malfunction (stuck close)
P1092 Tumble generator valve #2 (LH) malfunction (stuck open)
P1093 Tumble generator valve #2 (LH) malfunction (stuck close)
P1094 Tumble generator valve circuit #1 (open circuit)
P1095 Tumble generator valve circuit #1 (over current)
P1096 Tumble generator valve circuit #2 (open circuit)
P1097 Tumble generator valve circuit #2 (over current)
P1100 Starter Switch Circuit Malfunction
P1101 Neutral Position Switch Circuit High Input (A/T)
P1101 Neutral Position Switch Circuit Malfunction (M/T)
P1102 Pressure Sources Solenoid Valve Circuit Malfunction
P1103 Pressure Sources Switching Solenoid Valve Circuit
P1104 Engine Torque Control Signal Circuit Malfunction
P1106 Engine Torque Control Signal 2 Circuit Malfunction
P1110 Atmospheric pressure sensor low input
P1111 Atmospheric pressure sensor high input
P1112 Atmospheric pressure sensor range/performance problem
P1115 Engine Torque Control Cut Signal Circuit High Input
P1116 Engine Torque Control Cut Signal Circuit Low Input
P1120 Starter Switch High Input
P1121 Neutral Position Switch Circuit High Input [MT Vehicles]
P1121 Neutral Position Switch Circuit Low Input [AT Vehicles]
P1122 Pressure Sources Switching Valve Circuit High Input
P1124 TCS Signal Circuit High Input
P1130 Front oxygen sensor circuit malfunction (open circuit)
P1131 Front oxygen sensor circuit malfunction (short circuit)
P1134 Front oxygen (A/F) sensor microcomputer problem
P1137 Front oxygen (A/F) sensor circuit range/performance problem
P1139 Front oxygen (A/F) sensor #1 heater circuit performance/range problem
P1141 Mass Air Flow Sensor Circuit High Input
P1142 Mass Air Flow Sensor Circuit Low Input
P1143 Pressure Sensor Circuit Low Input
P1144 Pressure Sensor Circuit High Input
P1146 Pressure sensor circuit range/performance problem (high input)
P1150 Front Oxygen Sensor Heater Circuit High Input
P1151 Rear Oxygen Sensor Heater Circuit High Input
P1230 Fuel pump control unit malfunction
P1244 Wastegate control solenoid valve malfunction (low input)
P1245 Wastegate control solenoid valve malfunction (fail-safe)
P1301 Fire due to increased exhaust temperature
P1312 Exhaust temperature sensor malfunction
P1325 Knock Sensor Circuit Low Input
P1400 Fuel Tank Pressure Control Solenoid Low Input
P1420 Fuel Tank Pressure Control Solenoid High Input
P1421 Exhaust Gas Recirculation Circuit High Input
P1422 EVAP Purge Control Valve Circuit High Input
P1423 EVAP Vent Control High Input
P1440 Fuel Tank Pressure Control System Low Input
P1441 Fuel Tank Pressure Control System High Input
P1442 Fuel Level Sensor Circuit Range/Perf
P1443 EVAP Control System Vent Control Function Problem
P1480 Cooling fan relay 1 circuit high input
P1500 Radiator Fan Relay 1 Circuit Malfunction
P1501 Idle Control System Malfunction (Fail Safe)
P1502 Radiator Fan Function Problem
P1507 Idle Control System Malfunction (Fail Safe)
P1510 Idle Air Control Solenoid Signal 1 Circuit Low Input
P1511 Idle Air Control Solenoid Signal 1 Circuit High Input
P1512 Idle Air Control Solenoid Signal 2 Circuit Low Input
P1513 Idle Air Control Solenoid Signal 2 Circuit High Input
P1514 Idle Air Control Solenoid Signal 3 Circuit Low Input
P1515 Idle Air Control Solenoid Signal 3 Circuit High Input
P1516 Idle Air Control Solenoid Signal 4 Circuit Low Input
P1517 Idle Air Control Solenoid Signal 4 Circuit High Input
P1518 Starter switch circuit low input
P1520 Radiator Fan Relay 1 Circuit High Input
P1540 Vehicle Speed Sensor Malfunction 2 P1544 High exhaust temperature detected
P1560 Back-Up Voltage Circuit Malfunction
P1590 Neutral position switch circuit high input
P1591 Neutral position switch circuit low input
P1592 Neutral position switch circuit (MT model)
P1594 Automatic transmission diagnosis input signal circuit malfunction
P1595 Automatic transmission diagnosis input signal circuit low input
P1596 Automatic transmission diagnosis input signal circuit high input
P1698 Engine torque control cut signal circuit low input
P1699 Engine torque control cut signal circuit high input
P1700 Throttle Position Sensor Circuit Malfunction (A/T)
P1701 Cruise Control Set Signal Circuit Malfunction (A/T)
P1702 Auto Trans Diagnosis Input Signal Circuit
P1703 Low clutch timing control solenoid valve circuit malfunction
P1704 2-4 Brake Timing Solenoid Valve Circuit Malfunction
P1705 2-4 Brake Pressure Solenoid Valve (Solenoid D) Circuit
P1711 Engine torque control signal 1 circuit malfunction
P1712 Engine torque control signal 2 circuit malfunction
P1722 Auto Trans Diagnosis Input Signal High Input
P1742 Auto Trans Diagnosis Input Signal Malfunction

Links:
Subaru DTCs at autorepair.about.com
article at TroubleCodes.net/Subaru/

last updated 4 Mar 2008