Pressure switch won't shut compressor off (runs past cut-out, safety valve pops)

CheckLOTO. Remove the switch cover. Inspect contacts for pitting, burning or welding. With power off and the switch tripped (or pressure removed), check continuity across each pole: if it still shows closed when it should be open, the contacts are welded.

FixReplace the pressure switch. Then find why the contacts welded: a motor too big for the switch's HP rating (direct-switching a 5 HP motor through a switch rated 3 HP), low voltage causing high current, or frequent short-cycling. Above about 3 HP single-phase or any 3-phase unit, the switch should pilot a magnetic starter rather than carry motor current.

CheckMost switches sense pressure through a small port or tube on a manifold/tee. Rust, sludge or ice can plug it so the diaphragm never sees tank pressure. Drain the tank, remove the switch, and look into the port — blow through it. Common on tanks that are rarely drained.

FixClean the port and tee; replace rusted tank fittings. Start a daily drain routine or fit an auto drain.

CheckWatch the tank gauge (verify it with a known-good test gauge — tank gauges are often 10–20 psi off). If the switch opens slightly above the safety valve rating, the valve pops first.

FixTurn the main range nut down (counter-clockwise on most Square D/Condor switches) until cut-out is at least 10% below the safety valve set pressure, and within the tank and pump rating.

CheckAir leaks from the switch housing vent or under the cover, and the switch never trips.

FixReplace the pressure switch.

CheckOn units where the switch pilots a starter: switch opens (coil voltage drops to 0) but the contactor stays pulled in.

FixReplace contactor; check for undersized contactor or high inrush/low voltage.