Auto scaling with AMI

Created one EC2 instance from Amazon AWS. After did the changes on the instance, make one AMI from it.

Using the AMI, I have created on Auto scaling group to create one instance When CPU hit 70 %. Once done the Auto scaling creation, its create one instance at the end. So I deleted the Old one and move the traffic to the new instance.

but the problem here is , Auto scaling is not working when the CPU hits a threshold limit. Should I need to keep the Old machine which I used to create a AMI image ?.

Can anyone give your suggestion on this ?.

Thanks.