I have a older nordyne split unit heat pump... a few months ago the fan on the inside unit went out and I replaced it... everything worked fine for a couple weeks... now the outside unit comes on intermittently... I have replaced the contact switch, capacitor and thermostat... if I turn everything off and switch all the breakers to the unit off and back on sometimes it will kick on for a short time but then goes off again... any ideas or suggestions?

Are running in heat mode?

Outside unit won't run on ac or heat... auxiliary heat comes on inside

If you have a voltmeter I would check to see if you have 24V on the contactor outside. In the pics I am showing a single pole and double pole contactor. On both, the 24V will be on each side.

When checking the coil you can pull the disconnect thats next to the outside unit. Leave the furnace power on. This is the safest way to do it.

Have 24v... had a HVAC person come troubleshoot it.. he said it was the defrost board... wanted $500 to replace... Went to parts store got same board for less than $100... but still not working.

What's the model # of the defrost board?