Choosing the right contactor
Coil voltage first. Every contactor here uses a 24 VAC coil, the standard for residential and light commercial systems driven from a control transformer. If your existing coil is 120 V or 240 V, these are not a match — check the coil marking, not the line voltage.
Pole count follows the equipment, not the phase. A 1/1.5-pole switches one leg and is common on residential single-phase condensers where the other leg stays connected. A 2-pole breaks both legs, which is what most modern single-phase units and many local codes call for. A 3-pole is for three-phase equipment. Replacing a 2-pole with a 1-pole leaves a leg energised at the compressor — always match or exceed what came out.
Amperage: match or go higher. A 40 A contactor will happily carry a 30 A load and run cooler doing it. Going the other way overheats the contacts and welds them shut, which is the failure that leaves a compressor running after the thermostat is satisfied.