Why is my air compressor running but not building pressure?

A compressor that won't start announces itself. A compressor that runs happily while plant pressure sags is sneakier — and more expensive, because you're paying for every kilowatt while getting none of the air. The controllers have codes for it: Kaeser logs "No pressure buildup" (0044 A), Ingersoll Rand's Intellisys flags low sump pressure conditions, Sullair posts low line pressure and failed-to-unload faults, and Atlas Copco's instruction books document the runs-but-won't-load condition. Here's the short list, in the order that finds the cheap answers first.

1. Demand simply exceeds supply

Before blaming the machine, ask what changed in the plant. A new production line, an added blast cabinet, a big hand-tool crew on overtime — or the slow bleed of accumulated leaks — can outrun a compressor that's performing exactly to spec. If the machine loads, runs flat out, and pressure still sags, you have a capacity or leak problem, not a compressor fault.

2. Leaks — the silent 20-30%

Compressed air auditors routinely find plants losing a quarter of their air to leaks: failed quick-disconnects, cracked poly lines, condensate drains stuck open, abandoned drops that were never capped. Walk the system when the plant is quiet and listen; an ultrasonic leak detector finds the rest. Every leak you fix is capacity you already paid for. Several service companies on our Nashville and Birmingham pages offer formal air audits that quantify exactly this.

3. The machine isn't actually loading

A rotary screw only makes air when its inlet valve opens and the machine loads. If the controller shows the unit running but never entering LOAD — or the load solenoid never clicks — the problem is in the control chain: solenoid valve, inlet valve stuck closed, or (a documented Atlas Copco cause) a phase-sequence relay that opened after electrical work reversed rotation. Listen for the load transition; its absence localizes the fault immediately. The components themselves are technician territory.

4. The minimum pressure/check valve and separator

Between the machine and your plant sits a minimum pressure/check valve that keeps the sump pressurized, and the air/oil separator element the air must pass through. A defective check valve is a documented cause of pressure-buildup failures on Kaeser machines; a plugged separator chokes flow and shows up as high differential pressure — which is why the controllers monitor separator dP and warn about it. If your separator service is overdue and pressure performance has slipped, those two facts are probably one fact.

5. Reading mismatch — trust, but verify

Before a major diagnosis, compare the controller's pressure reading against a mechanical gauge (Kaeser's own service manual suggests exactly this cross-check against the separator tank gauge). A failed pressure transducer can make a healthy machine look sick — most brands display a specific sensor fault when the signal goes fully out of range, but a drifting sensor can lie without tripping anything.

What to have ready for the service call

The exact code or message, the machine's rated pressure and current setpoints, whether the unit audibly loads, what the mechanical gauge says versus the display, and what changed in the plant recently. That one paragraph of information routinely saves a diagnostic visit. Look up your controller's exact fault first — Atlas Copco, Ingersoll Rand, Kaeser, Sullair — then call.