Why Your Danfoss Pressure Switch Keeps Failing (It’s Probably Not the Switch)

I’m a controls tech who’s been taking Danfoss pressure-switch calls for 9 years. I’ve also personally made and documented 23 significant mistakes in pressure-switch troubleshooting. The worst one cost $4,600 and three days of downtime.

Here’s my confession: for the first few years of that career, I treated Danfoss pressure switches like cheap fuses. One fails, you replace it. It fails again, replace it again. Then you start ordering spares in bulk and wondering why the client is looking at you funny.

It took me about three seasons and 40-some replaced switches to admit the obvious: most of those switches weren’t broken. The problem was the system around them.

Another Failed Switch

In 2019, I was supporting a gas turbine power station contract. The lubricating oil line kept tripping on low pressure, and the maintenance supervisor had already swapped three Danfoss KP1 pressure switches in a month. He wanted to replace the entire lube oil pump next.

I almost agreed. That was the scary part. I’d worked a long shift, the switch was showing an intermittent fault, and my first instinct was to quote a new switch and bill the overtime.

Then I actually read the datasheet. Danfoss has multiple families for a reason: KP for general regulation, RT for industrial safety and high-pressure, MBC for applications with automatic reset. The KP1 in that cabinet was being used as a low-pressure alarm for a system that fluctuated by 8 bar during startup. The switch was doing exactly what it was designed to do.

It just wasn’t the right switch for the job.

What I Actually Found When I Stopped Blaming the Switch

After digging into the whole system, three things emerged. None of them were faulty switches.

1. The Air Filter Was the Real Culprit

In a gas turbine power station, the air filter for the turbine intake does a brutal job. It catches dust, sand, even pollen. But when the filter gets clogged, the pressure differential across it climbs. That upstream condition eventually creates pressure swings downstream that the switch sees as abnormal.

We had a clogged filter that hadn’t been serviced in six months. The filter was causing the lube oil system pressure to drop during peak loads. The switch was just the messenger.

That’s a lesson I didn’t learn until I’d already replaced a switch on a unit where the real fix was a $60 filter replacement. The “faulty” Danfoss KP1 pressure switch didn’t fail at all.

2. The 10-Circuit Manual Transfer Switch Was Lying

On the electrical side, the client’s control panel had a 10 circuit manual transfer switch that controlled the main and standby pumps. During a transfer, the load broke contact on one circuit for a few hundred milliseconds. That was enough to make the pressure switch circuit drop out and trigger a nuisance alarm.

The senior tech had actually mentioned the transfer switch on our first call. I dismissed it because the switch was showing an open circuit. He was right; I wasn’t.

Key point: a pressure switch can only report what it sees. If the electrical supply is interrupted at the wrong moment, the switch looks at fault but the problem is upstream in the wiring.

3. I’d Picked the Wrong Series

It’s tempting to think all Danfoss pressure switches are interchangeable because they share the same threaded ports. They’re not. The KP series is designed for general industrial applications, while the Danfoss RT industrial pressure switch is a different animal — rated for higher pressure, more cycles, and harsher conditions. If you put a KP1 on a 40-bar steam compressor that cycles 20 times an hour, you’re going to see early wear and drift.

For industrial high-pressure setups, the Danfoss RT pressure switch is usually the right starting point. That’s not just my opinion — Danfoss’s own selection guides make the distinction pretty clear.

4. And Yes, in Furnaces It’s Often How the Air Filter Was Installed

Away from the power station, I’ve also dealt with residential and light commercial furnace calls where the pressure switch won’t reset. Most of those turn out to be about how to install the air filter in the furnace or how often to change it. If the filter is dirty, the blower can’t move enough air, the differential pressure drops, and the combustion pressure switch locks out the furnace.

So even in heating, the pressure switch is telling the truth. The filter is the problem.

The Price of Not Looking Deeper

That one gas turbine project cost me more than just a weekend call. Let me break it down:

• Swapped three KP1 switches at about $110 each — $330 in parts.
• Labor and callouts: roughly $900.
• Downtime: two partial production days, which the client calculated as lost revenue of almost $3,000.
• Client confidence: priceless, but negative.

In total, that one “switch failure” cost north of $4,000 before anyone looked at the air filter. The filter and transfer switch fix cost about $140 in materials and one afternoon of my time.

The habit of assuming the pressure switch is bad is an expensive, repetitive mistake. I’ve also made the opposite mistake — replacing an old switch and calling it a “safety upgrade,” when the real reason was that I didn’t want to crawl under a machine to check the filter.

How I Check Before Replacing a Danfoss Pressure Switch

Now my team uses a simple pre-replacement checklist. It takes about 20 minutes, and it has caught 47 potential errors in the past 18 months, including 11 switches that would have been unnecessarily replaced.

1. Measure the system pressure. Compare it against the switch set point. If the pressure is abnormal, the switch is probably fine.
2. Check differential pressure across air filters. In gas turbine stations, furnace intakes, anywhere there’s a blower or intake filter.
3. Verify the electrical path. Test that 10-circuit manual transfer switch, relay contacts, and terminal connections.
4. Confirm the Danfoss series fits the application. KP for standard duties, RT for industrial high-pressure environments, MBC when you need automatic reset.

And here’s the part that might sound soft, but I mean it: I don’t care whether you’re buying one switch or fifty. I’ve built some of my best supplier relationships by being the guy who says, “You don’t need a new switch. You need to clean your filter.” The architect who wanted one tiny switch for a test rig is still the operations manager who now orders entire batches from me.

Small doesn’t mean unimportant. Sometimes the smallest order teaches you the most important lesson.

Honestly, I’m still not sure why the MBC switch on that lube oil skid kept drifting after we fixed the filter. My best guess is vibration from the adjacent compressor, but the client never let me back in to test it. If you have a theory, I’m all ears. In the meantime, I keep telling every technician I train the same thing:

If you’re on your third replacement, stop replacing. Start looking.

Danfoss pressure switches are good tools. They’re just not magic. Give them the right system, the right set point, and the right circuit, and they’ll keep working for years. Give them a clogged filter or a dirty transfer switch, and you’ll be back in the panel next month, ordering another spare.

I’d rather you spend that time and money on something real.