If you have an electrical meter, test across each element (one meter lead touching terminal at each end of element) for 240+vac.
With t-stat calling for Heat, after 2 minutes, you should read 240 vac across each element.
IF you don't, then most likely a bad Sequencer. If you do, further testing of element is needed.
On the Sequencer, the top and middle silver terminals are the M terminals......M1/M2, M3/M4, M5/M6.
The bottom brass terminals are the H terminals.....H1/H2, H3/H4 .
When t-stat calls for Heat, you should read 24-28vac across the H terminals (bottom brass).
IF you have 24-28vac at those, then you should read 0 vac across all the M terminals AFTER 2 minutes.
IF you read 240+vac across any of the sets of M terminals, the Sequencer is bad.
The Sequencer can be seen and ordered at below link if needed.
It is Sequencer # 3115-3571 @ $39.99 + $6 shipping.
Please note that heating elements for the 3400 series electric furnaces are no longer available. Restring kits are available to repair your existing heating elements.
Take care and best wishes,