A safety circuit is preventing the compressor from starting -- high temperature lockout, open door switch, low oil level, or a faulty sensor giving a false alarm.
What you'll see
Check for specific fault messages on the controller display. If it says 'Emergency Stop', see that page. If it says 'Motor Overload', see that page. This page covers the other safety circuits: temperature, pressure sensor, oil level, door switches, and similar interlocks.
Could also be:
How to diagnose
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Read the controller fault message
The controller will almost always tell you what's wrong. Read the actual fault code or message. On Atlas Copco machines, the 4 typical safety stops are: element temperature, defective pressure sensor, motor overload, and emergency stop. Each has its own fix.Result: Specific fault message = follow that diagnostic path. -
Check high temperature lockout
If the compressor was running and shut down on high temperature, it may need to cool down before it will restart. Some controllers have a lockout timer. Also: the temperature sensor itself can give false readings -- a loose connection makes the reading jump around. If the sensor cable has vibration damage, the controller might see a temperature spike that never actually happened.Result: Genuine high temp = fix the overheating. Sensor fault = check connections, replace sensor if faulty. -
Check door / panel switches
Some compressors have switches on the access doors or enclosure panels. If a panel is removed or not fully latched, the switch opens and the controller won't start the machine. Check that all panels are in place and the switches are making contact.Result: Open panel switch = close the panel or check the switch alignment. -
Check oil level interlocks
Some machines have a low oil level switch that prevents startup. Check the oil level -- is it below minimum? Top up if needed. If the level is fine but the switch is triggered, the switch itself may be faulty.Result: Low oil = top up. Normal oil + triggered switch = replace the switch. -
Check for phantom faults from bad connections
When a compressor stops randomly with no clear alarm, it's often a bad relay or loose connection inside the electrical cabinet. The safety circuit briefly opens due to vibration or a marginal contact, the controller sees a fault and shuts down, but by the time you read the display the connection has re-established. Check all alarm input wiring for tightness.Result: Loose connections found = tighten and secure.
How to fix it
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Address the specific interlock condition
Fix the root cause: clean the cooler if overheating, top up oil if low, close access panels, fix the pressure sensor. Then reset the fault on the controller and restart.
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Replace faulty sensors
If a temperature sensor or pressure transducer is giving erratic readings, replace it. These sensors are not expensive and a faulty one will keep causing nuisance shutdowns.
Don't disable safety interlocks to 'get the machine running'. If the high-temp sensor keeps triggering, fix the overheating -- don't disconnect the sensor. I've seen machines run to destruction because someone bypassed a safety switch. Also: intermittent faults that disappear when you look at the machine are almost always loose wiring, not ghosts.
Replacement temperature sensor, pressure transducer, or door switch depending on the fault. Multimeter for testing sensor outputs. Controller manual for fault code reference.
Safety interlocks exist for good reasons. Never bypass them permanently. If a high-temperature interlock is active, the machine IS hot -- let it cool down before investigating.