This is an old GE electric furnace. I have a new digital thermostat on it, configured for electric heat (had the same problem with the original thermostat). It will shut on and off perfectly for a few days, then I'll find that it's 85 degrees inside and the thermostat is set to 68, the furnace just cranking away. I replaced the fan relay (someone called it an ice cube relay?) and it worked ok for 2 weeks, now it's doing it again. The only thing left to replace is the sequencers, but how do I check their operation? I can't make the problem happen; it's intermittent.

Thanks in advance for the help

Sequencer only needs to be replaced for the one that turns the fan off and on for heating. Follow the wiring diagram on your unit to see where the sequencers are located. Find the connections that operate off of 24 volts and attach probes. Turn off thermostat and see if the voltage drops at the sequencer. If the sequencer stays on you found the problem. If not remove on leg of the low voltage wires from the thermostat from the heat circuit. This will cut off the thermostat control. This will do just about the same thing.

You can also check the thermostat by hooking up the meter to the 24 volt heat control wires. With no call for heat by the thermostat there should be no 24 volts showing.