Fuel Filter Restriction High Pressure Drop Detected
The SULLAIR-E031 (Sullair WS Controller) diesel fault code means: Fuel Filter Restriction High Pressure Drop Detected. This is a moderate severity code.
- Keep driving?
- Yes, but fix soon
- DIY difficulty
- easy
- Estimated cost
- DIY filter replacement $40-$120 in parts depending on engine model. Professional service including filter replacement, system bleed, and switch inspection typically $150-$350 at a Sullair dealer or rental service shop. Replacement restriction switch alone is $25-$75 in parts if the switch has failed.
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Common Symptoms
- WS Controller LCD displays E031 and fuel filter restriction warning on the side panel
- Engine may run rough or surge at high load demand
- Compressor output pressure drops below setpoint under load
- Engine may derate RPM or shut down on repeated fault trips
- Yellow or red fault indicator illuminates on the controller panel
- Engine is harder to start or cranks longer before firing
- Machine shuts down under full load but restarts fine at idle
Probable Causes (Ranked by Likelihood)
- Primary fuel filter clogged from normal service interval overrun Very Likely
- Secondary fuel filter plugged with debris or water contamination Very Likely
- Water-in-fuel separator bowl full and not drained, restricting flow Likely
- Contaminated or low-grade diesel causing accelerated filter loading Likely
- Fuel restriction switch failed closed or wiring fault to WS Controller Possible
- Fuel supply line kinked, collapsed, or partially blocked upstream of filters Possible
- Cold weather causing wax or gel buildup in filters and fuel lines Less Likely
Step-by-Step Diagnostic Procedure
Step 1. Note the E031 code on the Sullair WS Controller LCD. Write down how many hours are on the machine and compare to the last filter service entry in the service log. Sullair recommends primary and secondary fuel filter replacement every 500 hours or annually, whichever comes first.
Step 2. Shut the machine down using the STOP button and allow it to cool for 5 minutes. Locate the primary fuel filter and secondary fuel filter on the engine. Check both filter housings for obvious swelling, leaks, or external contamination.
Step 3. Drain the water separator bowl. Most Sullair towable units have a clear or translucent bowl at the base of the primary filter head. Open the drain petcock and check for water. Water separates as a darker layer at the bottom. If you see significant water, fuel contamination is likely the root cause.
Step 4. Inspect the fuel supply line from the tank to the filter inlet. Look for kinks, crush points, or signs that the line has been driven over. A partially collapsed line can mimic a plugged filter on the WS Controller restriction switch.
Step 5. Replace both the primary and secondary fuel filters with Sullair-specified or OEM-equivalent elements. Bleed the fuel system per the engine OEM procedure. For John Deere engines, crack the bleed screw on the secondary filter head and run the lift pump until fuel flows bubble-free. For Cummins QSB and CAT engines, use the priming pump on the filter head.
Step 6. After filter replacement, start the machine in MANUAL mode and observe the WS Controller LCD. If E031 clears and does not return under load, the filters were the cause.
Step 7. If E031 returns immediately after new filters are installed, locate the fuel restriction switch. It is typically a normally-open pressure differential switch plumbed across the primary filter. Disconnect the two-wire harness connector and use a multimeter set to continuity. The switch should read open circuit with clean filters installed. If it reads closed with new filters in place, the switch has failed and needs replacement.
Step 8. If the switch checks out and the code still returns, connect the appropriate engine OEM tool (JD Service Advisor for John Deere, CAT ET for Caterpillar, Cummins INSITE for Cummins) to check for any engine-side fuel system DTCs that may be contributing to the restriction indication.
Frequently Asked Questions
What does Sullair code E031 mean?
E031 means the WS Controller has detected a high pressure differential across the fuel filter circuit, which tells you the primary or secondary fuel filter is restricted enough to limit fuel flow to the engine. The most common cause is a filter that is overdue for service.
Can the compressor still run with code E031 active?
Usually yes, but with reduced reliability. The engine may run rough, lose power under full load, or eventually shut itself down if fuel starvation gets severe enough. You should not run the machine hard with this code active. Replace the filters before putting it back into heavy service.
How much does it cost to fix E031?
If it is just clogged filters, a DIY fix runs $40 to $120 in parts depending on the engine. A service shop will typically charge $150 to $350 for filters, bleed, and inspection. If the restriction switch itself has failed, add $25 to $75 for the switch plus labor.
Will E031 come back right after I replace the filters?
If the filters were genuinely plugged, replacing them and properly bleeding the fuel system should clear the code. If E031 returns right away with fresh filters installed, the fuel restriction switch has likely failed closed and needs to be replaced. Check it with a multimeter before ordering parts.