I have an older york heatpump,
Problem is sometimes, when heat is on thermostat will kick off,

Fan will keep blowing, but cold air comes out.

Thermostat emergency heat light flashes 4 times then pause then 4 times etc.

Only way to fix is trun to E heat, or turn off heatpump a few times,

Sometimes it will work correct for days other times only about 1/2hr.

Always does the 4 time flashing of E light when it messes up

What kind of tstat, air handler and HP are we dealing with? Know how to use a voltmeter?