I OWN A 2001 SUBARU LEGACY GT. I STARTED TO SMELL GASOLINE IN THE PASSENGER AREA WHEN THE HEATER IS TURNED ON. WHAT CAN CAUSE THIS PROBLEM ?
You obviously have a fuel leak. The fumes are being sucked into the passenger compartment when the heater fan comes on. It leaks the rest of the time too. Look under the hood with the motor running to find it. Do not smoke during this process. Smoking can be hazardous to your health. So can not fixing it.
Kindly do not post in all caps. It is hard to read and internet etiquitte (spellcheck?) considers it “shouting”.
in all likelyhood you have a heater core problem. (leak)
is there a haze or film on the window above the defrost vents?
sometimes people confuse the smell of antifreeze and gas.
check the level of the radiator overflow bottle make sure the level is between the full and low marks.
MG McAnick and Cappy have both made good points. While I tend to lean toward a gasoline leak, it is possible that you are perceiving the smell of an antifreeze leak to be the smell of a gasoline leak.
Neither scenario is good, and you need to have this resolved sooner, rather than later, simply because you have potential health and safety issues here. If there is no “haze” on the inside of your windshield and if your antifreeze level is normal, then it is probably a gasoline leak. Get it fixed before your car (and possibly its passengers) disappears in a ball of flame.