Subaru 2003 Legacy Owner's Manual page 109

Keys and doors
the transmitter.
B Tripped sensor identification
If an electronic chirp sounds four times and the turn signal lights flash
four times when you disarm the system, this indicates that the alarm
was triggered. The number of times the indicator light flashes indi-
cates what sensor caused the alarm condition.
2
To enter identification mode:
1. Open the driver's door and leave it open.
2. Turn the ignition switch to the "ON" position two times. ("LOCK"
"ON" "LOCK" "ON" "LOCK")
3. The indicator light will start flashing.
The indicator light provides the following indications.
Flashing one time: Any door, the rear gate or the trunk lid has been
opened.
Flashing three times: The impact sensor in the system's unit has
been activated. This may indicate that your vehicle has been shocked
or tampered with by some outside force and/or unauthorized person.
Flashing four times: The ignition switch has been turned on or the
battery has been disconnected.
NOTE
The battery must be reconnected before the indicator light will
flash.
To exit this mode, close the door and turn the ignition switch to the
"ON" position.
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