Is this right ? congress had to decide the outcome of the Election of 1800 because no candidate won the electoral vote. When that happens, the

Constitution mandates that they must decide the outcome instead of the electors

Yes, that is correct. In the Election of 1800, no candidate won a majority of the electoral vote, which triggered the process outlined in the Constitution. According to the Constitution, when no candidate receives a majority, the House of Representatives must decide the outcome from the top candidates. In this case, Congress had to decide the winner between Thomas Jefferson and Aaron Burr.