How to Check a Contactor

• Coil voltage is printed on the contactor: 24 VAC is typical for residential HVAC, while 120, 208/240, and 480 V coils are common in commercial work.
• Number of poles (1, 2, or 3), the FLA/resistive rating, and the LRA rating must suit the load.

• Pitted, burned, or welded contacts. Melted or discolored line or load lugs, which point to loose terminations. Check torque.
• Insects (ants are attracted to the contacts), debris, or rust that stop the armature from seating.
• A burned or cracked coil, or brittle wiring.

1. Control voltage: with a call for cooling, measure across the coil terminals. You should see the rated coil voltage (for example about 24–28 VAC). No voltage means the problem is upstream: thermostat, control board, transformer, a safety switch, or a blown fuse.
2. Voltage present but the contactor won't pull in means a bad coil or a stuck armature. Low control voltage, below about 80–85% of the coil rating, causes buzzing or chatter. Check the transformer and look for voltage drop in the thermostat wire.
3. Coil resistance (power off, one coil wire lifted): OL or infinite means an open coil. Near 0 Ω means a shorted coil. Compare against the manufacturer's value or a known-good contactor of the same model.

1. With the contactor pulled in and the load running, measure across each pole, line terminal to load terminal (L1–T1, L2–T2).
2. Good contacts read close to 0 V. More than a few tenths of a volt means pitted or high-resistance contacts. That voltage becomes heat (V × A watts), so replace the contactor.
3. With the contactor open, the reading across an open pole will be close to line voltage. That is normal.
4. Compare load-side voltages on each leg. They should match the line side.

The same coil voltage and the same or more poles, with FLA and LRA ratings at or above the original. Match how the control wires terminate. Re-torque all lugs.