'99 Accent - check engine + dash lights + beeping sound

Hello,

I have a 99 Accent, 1.5L manual transmission.

My check engine light came on a while back, I scanned it, and I then changed, as stated, the read O2 sensor. It came on again about 3 weeks later - I have yet to scan it again.

On that note, I notice once in a while the parking brake, and battery lights light up at the same time, not fully - just dim, for short intervals. The old owner changed the alternator for a small crappy one just before I bought it.

Also, any way to get my remote lock to work? The battery is good, but the car does nothing when I press it.

Finally, every time I turn off my car, a beeping noise goes off - 3 beeps a number of times. This also occured to me today, during driving, and the radio and ventilation lost power at the same time 9for aobut 30 mins)

Thank you so much