My unit had the main circuit panel replaced 2 weeks ago. (2nd time in 5 years). Not sure why keeps going out, but we have a lot of power surges and lightening problems here. Anyway, now, the contactor which engages the compressor outside won't stay in so that the compressor will kick on unless the unit has been turned off for about 30 minutes. It also appears that it has burn marks on it. We can press and hold the plunger down and it will then stay, of course, until the thermostat cuts it off. Then when the temperature insude rises again then we have to go back outside and press it back down and hold for a minute again. Will simply replacing the contactor be okay and if so where can I find one the cheapest.

Make sure you are getting 24 volts to the contactor when it is supposed to start/activate. If you are not getting the required 24 volts then you will have to track down the voltage problem BUT if you are getting the 24 volts then just replace the contactor and that should get your system up and running properly.