I have a intertherm air conditioner, single out side unit. Starting last summer I have replaced the start reley,transformer all ran well when replaced, I went to turn on the air this week and nothing, just made a humming noise at the unit out side. So I replaced the run capacitor, results same, no air. But I can hold the start reley button in and it runs and let go and it turns off.no burnt wires so I'll check my t-stat. any ideal's

If the contactor is not closing its going to be a low voltage problem. Jump R to Y at the thermostat, no A/C, jump R to y inside the unit, no A/C, you have a time delay or pressure switch that is open for a reason. If you can give make and model of your system it will help.