My chevy aveo 5 has its check engine light on. I tried checking and tightening the gas cap but the light is still on.

I do an oil change on the vehicle and within 2-3 weeks the oil level has dropped to a point where it causes my car to stall or not excelerate. Like it looses all compression.

at the dealer say is because i
installed an after market alarm do you think it is the reason
of the failure ? . some times it works . when i turn on the ignition it jumps a little thankyou