ok so i just bought this 04 WRX from a guy in Arkansas, and the check engine light was on but he said all it was was the mass air flow sensor. so I got home and plugged it back and, and the light went off but it began to idle funny. it would be at 1000 fine and then dip down to about 800 or so and sit for a second, and then jump up to about 1200 and then dip back down again. i took it to the shop to get it registered and what not and it passed so i pulled the sensor back out, and now its idling fine again. anyone know why thats happening and is there any harm to leaving this sensor unplugged? if so how do i go about fixing it? thanks guys