Engine Air Filter Restriction Warning

Common Symptoms

• Sullair WS Controller LCD displays E040 on the side panel readout
• Engine exhaust turns black or dark gray under load
• Compressor output pressure drops or cannot hold set pressure under demand
• Engine RPM may hunt or feel sluggish when the service valve opens
• Yellow or amber fault indicator illuminates on the control panel
• Unit continues to run but trips a warning rather than shutting down immediately
• Air filter restriction indicator on the filter housing shows red if equipped

Probable Causes (Ranked by Likelihood)

• Clogged or saturated primary air filter element from dust, dirt, or debris ingestion Very Likely
• Faulty or stuck air filter differential pressure switch giving a false trip Likely
• Filter housing seal or end cap missing, allowing dust bypass and early plugging of the element Possible
• Operating environment with extremely high airborne dust, chaff, or particulate loading the filter faster than normal service intervals Possible
• Safety filter element (secondary) plugged, restricting airflow even after primary element replacement Possible
• Wiring fault or corroded connector on the differential pressure switch signal circuit to the WS Controller Less Likely
• Collapsed or kinked intake air hose between the air cleaner housing and the engine throttle body Less Likely

Step-by-Step Diagnostic Procedure

1. Check the air filter restriction indicator window on the filter housing first. If the indicator is in the red zone, the primary element is restricted and needs replacement before any other diagnosis.

2. Remove the primary air filter element and hold it up to daylight. If you cannot see light clearly through the pleats or the element is visibly loaded with dust, mud, or debris, replace it. Do not blow compressed air back through the element from the dirty side as a fix. Sullair recommends element replacement when restricted.

3. With the primary element removed, inspect the secondary safety element inside the housing. If it shows dust loading or deformation, replace it as well. Never run the engine without the safety element in place.

4. Inspect the air cleaner housing gaskets and the housing end cap O-ring. A missing or cracked seal lets dusty air bypass the filter and reach the engine, which also loads the element faster and can trip the switch early.

5. Check the intake hose between the air cleaner outlet and the engine air inlet. Squeeze and flex it along its length. A collapsed inner liner or cracked hose causes restriction even with a clean filter element.

6. Locate the differential pressure switch mounted on the air cleaner housing or intake tube. With the engine OFF and key OFF, unplug the switch connector and inspect the terminals for corrosion or pushed-back pins. Use a multimeter in continuity mode. With no restriction present and the element clean, the switch contacts should be open. If the switch reads closed with a clean filter installed, the switch itself has failed and needs replacement.

7. After replacing the filter element or correcting the wiring fault, clear the E040 warning on the Sullair WS Controller LCD using the STOP and reset sequence per your unit's operator manual, then restart and confirm the fault does not return under load.

8. If E040 returns immediately after installing a new element and the switch checks out electrically, escalate to a Sullair service tech to inspect the engine intake manifold for cracks or leaks that could indicate deeper airflow or boost issues on turbocharged models.

Frequently Asked Questions