Before you start — safety checks
Always isolate the compressor electrically (LOTO) and depressurise the system to 0 bar before opening any pressure-containing components. Verify zero-energy state with a calibrated pressure gauge — never rely on the control panel gauge alone.
- Verify the compressor is running (motor turning, inlet air audible)
- Check the discharge pressure gauge — is it reading zero, or is it rising slowly?
- Note whether the compressor is in loaded state (fully loaded or in partial load/modulating)
- Check for any active alarms or fault codes on the controller
Rotary screw compressor — fault diagnosis
Fault 1: Inlet valve (intake valve) stuck or not opening
Symptom: Compressor runs but air intake is minimal. Unit may reach operating temperature quickly without building pressure. Inlet filter vacuum gauge reads very high (if fitted).
Diagnosis: Remove the inlet filter. With the compressor running at no load, you should feel strong suction at the inlet. If suction is weak or absent, the inlet valve (also called the intake valve or butterfly valve) is suspect.
Cause: Carbon/varnish build-up on the butterfly disk, failed actuator, broken spring in valve assembly, or solenoid valve fault (on electronically controlled inlets).
Fix: Inspect and clean the inlet valve. Test the solenoid valve for correct voltage and function. Replace valve assembly if butterfly disk is damaged or actuator has failed.
Fault 2: Minimum pressure valve (check valve) failed open
Symptom: Pressure will not build above 4–5 bar. Compressor loads correctly but discharge pressure stays low.
Diagnosis: The minimum pressure valve (typically 4.5 bar cracking pressure) maintains receiver pressure above the oil separator operating threshold. If the MPV spring has failed or the valve is stuck open, compressed air escapes to the service line before separator pressure builds. Check pressure downstream of the MPV — if it reads the same as the separator pressure, the valve has failed open.
Fix: Replace the minimum pressure valve assembly.
Fault 3: Oil separator element blocked
Symptom: High differential pressure across the separator element (typically alarm at 1.0 bar ΔP). Compressor runs but capacity is reduced. May see high discharge temperature alarm alongside.
Diagnosis: Check separator ΔP gauge (fitted on most screw compressors). ΔP exceeds 0.5 bar indicates replacement due. Overdue service is the most common reason for this fault.
Fix: Replace the air/oil separator element as per service interval (typically 2,000–4,000 hours depending on oil type and duty). Use OEM or approved equivalent element.
Fault 4: Air leak in the system
Symptom: Compressor is building pressure but cannot maintain it — pressure drops rapidly when compressor off-loads. Or compressor runs continuously fully loaded and pressure never reaches cut-out.
Diagnosis: Leak test. With the compressor loaded and the service isolation valve closed, watch the pressure gauge. Pressure should hold steady. If it drops more than 0.5 bar in 5 minutes, there is a significant leak. Conduct an ultrasonic leak survey. See our compressed air leak detection guide and leak cost calculator.
Fault 5: Pressure switch or transducer fault
Symptom: Compressor loads and unloads at incorrect pressures, or does not reach setpoint even though mechanical pressure seems correct.
Diagnosis: Compare controller pressure reading against an independent calibrated gauge screwed into the service manifold. If readings differ significantly, the transducer is suspect.
Fix: Calibrate or replace the pressure transducer.
Reciprocating (piston) compressor — additional faults
Fault 6: Worn piston rings
Symptom: Single-stage: compressor fills slowly. Two-stage: compressor builds stage-1 pressure but cannot reach final pressure. Excessive oil consumption. Blue smoke from inlet (oil blow-by).
Diagnosis: Perform a compression test. With the compressor running at low load, measure inter-stage pressure and final pressure against specifications. Low inter-stage pressure on a two-stage unit points to worn first-stage rings.
Fix: Overhaul — replace piston rings. Check cylinder bore for wear and scoring; reline if beyond wear limits.
Fault 7: Faulty reed valves
Symptom: Compressor runs but pressure builds slowly or not at all. Tapping or chattering noise from the valve plate. Hot head temperature.
Diagnosis: Remove valve plate assembly. Inspect reed valves for cracks, bending, carbon deposits, or stuck-open condition. A broken reed valve on the discharge side is the most common cause — compressed air recirculates back into the cylinder instead of exiting.
Fix: Replace the valve plate assembly or individual reed valves. Clean the valve plate and seating surfaces.
Fault 8: Unloader valve not closing
Symptom: Compressor "unloads" immediately on start-up and stays unloaded. Pressure will not build. Audible air blow-off from the unloader port.
Diagnosis: The unloader valve releases cylinder pressure during start-up and when the pressure switch trips off. If it fails to close once the compressor is running, the compressor runs under no load and cannot build pressure.
Fix: Inspect the unloader solenoid valve for debris or failed coil. Check the unloader pilot line for blockage. Replace solenoid valve or pilot valve as required.
Diagnostic checklist summary
- Motor and drive running normally (check amps, no trips)
- Inlet air filter clean (ΔP within limits)
- Inlet valve/butterfly opens correctly on load signal
- No active fault codes or alarms on controller
- Minimum pressure valve function — cracking pressure correct
- Oil separator ΔP within limits (replace if >0.5 bar)
- Pressure transducer reading matches independent gauge
- Pressure relief valve seated and not leaking
- Service line isolation valve open
- No major air leaks in distribution system
- (Piston only) Reed valves intact, valve plate sealing correctly
- (Piston only) Piston rings not worn — compression test passed