If your Kaeser rotary screw compressor is showing a flashing code on the Sigma Control and production is waiting, this checklist is for you. It isn't a replacement for the manual (or for the training your Kaeser service rep can give you). It's the triage sequence I use when I get called to a plant that's down—either over the phone or in person. In my role coordinating emergency service for compressed air systems, I've seen Kaeser compressor error codes that looked terrifying and were actually just a loose wire—and I've seen minor warnings turn into expensive repairs because someone kept hitting reset. The old belief that any error code means the compressor is dying comes from an era when machines had simpler relays and no diagnostic memory. That has changed.
Before you do anything, take a photo of the display. Actually, take two: one wide shot of the entire control panel, one close-up of the code. Write down the code and the time. That one habit has saved thousands of dollars over the years. The most frustrating part of error codes? They're clues, not conclusions. A code doesn't tell you which part is bad; it tells you what the machine was doing when it complained.
The 7-Step Triage Checklist
I run these in order. Some steps are boringly simple—that's the point. By the time you've checked all seven, you'll either have found the issue or you'll have useful information for the service tech.
- Copy the code and note the operating condition. A code that appears while loaded means something different from a code at start-up. Scroll through the history on the Sigma Control. According to Kaeser's manuals (us.kaeser.com, accessed January 2025), error code lists vary by model and firmware, so don't rely on a generic internet code table. Use the table in your own manual.
- Check the electrical supply before you suspect the machine. Many emergency calls are tripped breakers or loose terminals. Use a multimeter and check voltage at the incoming disconnect. In March 2024, I took a call from a facility with a non-communicating compressor. They'd already ordered a new printed circuit board. After 20 minutes on the phone, I had them check line voltage at the terminal block—it was a loose wire. The board went back for a credit. That's not to say boards don't fail, but verify first.
- Check the intake filter and the compressor room. A clogged inlet filter can cause high discharge temperature and pressure faults. If your Kaeser has a filter restriction indicator, look at it. Then look around the room. I found a compressor in a small mechanical room where the 20x25x1 air filter for the room's HVAC supply was completely blocked. Replacing that facility filter fixed the overheating issue. Do not, however, use a 20x25x1 air filter on the compressor intake. The compressor needs the correct Kaeser inlet filter element, not a 1-inch HVAC filter that will collapse under vacuum and let particles through.
- Look at the separator element and oil system. High pressure differential across the air/oil separator is a common cause of service-due and overpressure warnings. If you're overdue on the separator, your Kaeser compressor maintenance schedule just moved up. A blocked separator also causes oil carry-over, which can coat downstream components. If you see a high-temperature alarm plus a maintenance reminder, don't reset the reminder without checking the separator.
- Don't forget what's downstream. Backpressure from a closed isolation valve, a bad check valve, or a faulty air dryer can appear as a compressor fault. If your package uses a Bendix air dryer—common on truck-mounted or retrofit utility compressors—check that it's purging correctly. A Bendix air dryer is a desiccant dryer: it removes moisture by cycling air through the desiccant bed and purging it to atmosphere. If the purge valve sticks, moisture and pressure issues can feed back into the compressor. Your Kaeser controller won't show a dryer code, so you have to look at the dryer itself.
- Check coolant level and use the right fluid. Low coolant causes high-temperature alarms, but do not just add any oil. Use Kaeser SIGMA fluid or whatever fluid is documented for your model. Mixing fluids can cause varnish, restrict the cooler, and lead to a much bigger repair.
- Reset once and monitor. If the code returns, stop resetting. One reset is okay. The second reset is denial. I still kick myself for a job in 2022 where I reset a service-due code three times instead of replacing the separator element. The result was a blown separator, oil all over the piping, and a cleanup bill that was ten times the cost of the element. If it repeats, the machine is trying to tell you something.
What Is a Heat Pump? (And Why It Matters Here)
I hear this question from managers when a compressor fails, so let's settle it. A heat pump is a device that moves heat from one place to another using a refrigeration cycle. It has a compressor in it, but that compressor is doing a different job. A heat pump compressor moves refrigerant to transport heat; a Kaeser air compressor compresses air to deliver a volume of compressed air at pressure. The pressures, controls, and fault logic are totally different. If you're used to heat pumps, don't apply those diagnostics to a rotary screw compressor. And if someone calls a Kaeser compressor a 'heat pump,' don't let that confuse your maintenance planning.
Three Field Mistakes I See
- Resetting codes instead of finding the root cause. Reset to confirm a code is intermittent, but don't reset to avoid downtime. The code knows.
- Replacing parts because someone on the internet says 'code X means sensor Y.' It might mean that, but it might mean the wiring to the sensor, the connected process, or the controller. Check before spending.
- Skipping scheduled maintenance because the machine runs fine. Kaeser compressor maintenance intervals are based on hours and load cycles. By the time it runs badly, it's already an emergency.
When to Call for Service
If you've gone through all seven steps and the error is still there, that is the time to call a qualified Kaeser service technician. It doesn't mean you failed. It means you did the work that makes their diagnosis faster. And faster diagnosis means less downtime—which, honestly, is the whole game.
A Note for Small Facilities
If you have one small Kaeser and a part-time maintenance person, don't feel pushed around by the 'you should have a service contract' speech. Small doesn't mean unimportant. I've seen a $200 repair turn into a $12,000 emergency because nobody wanted to check the basics first. If a service vendor treats your single compressor like it's not worth their time, find another vendor. The ones who respect small requests are the ones to keep for when the big project shows up.
