About a week ago, the E3 have triggered the Dispute Resolution Mechanism of JCPOA after Iran has announced yet another escalation of their non-compliance. (Germany in particular has confirmed that Trump threatened them with tariffs on autos if they didn't do this.)
As you might know, the US itself has withdrawn from JCPOA in May 2018 and has since then imposed a "maximum pressure" campaign of sanctions on Iran, which resulted in a 88% reduction of Iranian oil exports and in a shrinkage of Iranian GDP of 9.5% in one year.
Of course the E3 (and the EU) could threaten Iran with any number of sanctions of their own hoping to force Iran to return to compliance. (Also, Trump could again threaten the EU with tariffs and what not if they didn't impose some sanctions he wants, but that's besides the point here.)
JCPOA ultimately threatens Iran with reimposing all prior UNSC sanctions, in the so-called snapback mechanism.
STEP FIVE - Once the complaining party notifies the Security Council, the body must vote within 30 days on a resolution to continue Iran’s sanctions relief. A resolution needs nine votes in favor and no vetoes by the United States, Russia, China, Britain or France to pass.
STEP SIX - If such a resolution has not been adopted within 30 days, the sanctions in all previous U.N. resolutions would be re-imposed - referred to as snapback - unless the council decided otherwise. If the previous sanctions are re-imposed they would not apply retroactively to contracts Iran signed.
So basically if no relief-continuation UNSC resolution passes (it probably won't pass because the US can veto that), the old/suspended sanctions are reimposed automatically.
I'd like to know: are these [older/suspended] sanctions substantially worse than what Iran is facing already? Basically, what is the letter of JCPOA threatening Iran with that they aren't experiencing already?