What it means
The compressor did not unload when the controller commanded it to.
This entry is tied to the exact STS message family documented by Sullair. The same words can mean something different on another controller generation, so confirm the displayed message and machine documentation before acting.
Likely causes
- Inlet valve or unloading controls did not respond correctly
Operator-safe first checks
- Do not continue cycling the compressor if it cannot unload normally.
- Record line pressure, sump pressure and operating mode when the event occurs.
Trained maintenance / service checks
- Inspect inlet-valve operation and the associated control system.
- Verify solenoids, control air and machine-specific unloading components using the compressor operator manual.
Reset or keep running?
Do not use repeated resets as a substitute for diagnosis. If the condition involves electrical power, a motor overload, high pressure, loss of cooling, abnormal temperature, or a safety circuit, correct the underlying problem before returning the compressor to service.
Source confidence
OEM VERIFIED
This message and its troubleshooting direction are traced to the Sullair STS controller troubleshooting guide.
Source: Sullair STS Controller User Manual, part 02250241-178 R01. Referenced manual page: 31.