It's very unlikely that you will be able to obtain instant health insurance coverage in (or on the way to) the ER. Even though it prohibits denying coverage based on pre-existing conditions, the ACA doesn't require that coverage be provided immediately and has some provisions to reduce adverse selection. One way it does that is through open enrollment periods
. Unless there is a qualifying event (job loss, divorce, etc), in most cases individuals won't be able to obtain coverage outside of the annual open enrollment periods (the first period will last 6 months but subsequent ones will be much shorter) and instead have to wait until the next open enrollment period. Even if you needed emergency care during the open enrollment period and managed to send in an application and premium payment in (or on the way to) the ER, it's unlikely that the effective date of coverage would be the same day or even the next one. In a best case scenario, it might take 48 hours or even several business days.