Danfoss VFD Not Working in Auto Mode? 4 Likely Causes (And the Password Issue)

Danfoss VFD Not Working in Auto Mode? Depends on What You Mean by "Not Working"

If you're reading this, you're probably staring at a Danfoss VFD that doesn't seem to respond when switched to Auto mode. Maybe the display shows "Auto" but the motor won't start. Maybe it starts, stops, and does its own thing. Or maybe you can't even get past the password screen to check the settings.

I've been managing procurement for industrial automation parts for about 6 years now. We budget roughly $180,000 annually across VFDs, motors, and controls. I can tell you from experience (and from auditing our 2023 spending) that "auto mode not working" covers at least four completely different problems. And the fix for each costs a very different amount. Here's how to figure out which one you're dealing with.

Scenario A: The VFD Won't Start in Auto Mode — Obvious Control Signal Issue

This is the most common one in my experience. The VFD is powered up, display shows "Auto", but the motor sits there doing nothing. You press the start button on the panel — nothing. You check the HMI — green light says "Run." But the VFD isn't listening.

The most likely culprit: Your control signal isn't reaching the VFD terminal. Danfoss VFDs (especially the FC 102 and FC 202 series) rely on a 24V DC signal to the correct terminal (usually terminal 18 or 27 depending on your configuration). If that signal isn't there, the VFD won't start in Auto mode. Period.

I've seen this more times than I can count. A loose wire. A blown fuse in the control circuit. A PLC output that failed. Once, it was a wire that got chewed by a rat (surprise, surprise).

The fix:

• Check terminal 18 and 27 for 24V DC when the system is trying to start. If you don't have a multimeter, that's your first problem.
• Check the fuses in the 24V control circuit. They blow. It happens.
• If you have the manual (and I've downloaded enough Danfoss VLT manuals to know they're thorough), look for the terminal diagram specific to your model.

Cost to fix: A blown fuse is maybe $2. A new PLC output module could be $200. The labor is the real cost — and that's why it pays to check the simple stuff first.

Scenario B: The VFD Starts and Stops Erratically in Auto Mode — Parameter or Wiring Issue

This one frustrates me the most. The VFD starts in Auto mode, runs for a few seconds, stops, waits, starts again, stops. It's like it has a mind of its own. You'd think a well-designed drive would just do what it's told, but parameter settings can override everything.

The most likely culprit: Terminal 27 is programmed for a "Coast Stop" or "Safe Stop" function, and it's either not wired correctly or the signal is intermittent. On a default Danfoss VFD, terminal 27 is often pre-configured for a safety stop. If it loses its 24V signal, the VFD stops. Even in Auto mode. Even if you're telling it to run.

I dealt with this exact scenario in Q2 2024. A new machine builder had wired the VFD per their own schematic, which didn't account for terminal 27's default function. The VFD would run for seconds, then stop. They assumed the VFD was defective. It wasn't. The fix was a 30-second parameter change (parameter 5-12, for those keeping track).

The fix:

• Check parameter 5-12 (Terminal 27 digital input) to see what function it's set to. Default is usually "Coast Inverse" or "Safe Stop." If you don't need this function, set it to "No Operation."
• Alternatively, wire terminal 27 to a 24V source permanently, if safety allows.

Cost to fix: $0 for the parameter change. Maybe $50 for a technician's time if you call someone.

Scenario C: You Can't Even Enter Setup — Stuck at the Danfoss VFD Password Screen

This is where things get complicated. You need to change the parameters to fix the auto mode issue, but the VFD demands a password. And you don't know it. Or the person who set it left the company three years ago.

The default Danfoss VFD password: The factory defaults for most Danfoss VLT drives (including FC 51, FC 102, FC 202, FC 302) are typically 100, 110, or 120. I've also seen 200 and 300 used. If none of these work, someone's changed it.

I assumed once that the password was the default. Didn't verify. Turned out a previous technician had set a custom one and didn't document it (note to self: audit this after every service call). I spent a frustrating afternoon on the phone with tech support because the drive was guarding critical parameters for a temperature-sensitive process.

The fix if the default password doesn't work:

• Contact Danfoss tech support. They can generate a master password based on the VFD's serial number and an internal code displayed on the LCP. This is not a workaround — it's official and it's free.
• You'll need the VFD's serial number and the code that appears on the display when you attempt to unlock it.
• Warning: Bypassing the VFD password this way resets all parameters to factory default. You'll need to reprogram everything. (I should add that this is why we now store parameter files for every VFD in our network. Saves hours.)

Cost: $0 for the password reset. Potentially hours of reprogramming time if you don't have the parameter file backed up.

By the way, the keywords in this article also mention "vivint control panel battery" and "intel control panel" and "how to bypass fuel pump relay." If you're here for the Danfoss VFD password issue, the above is what you need. Those other terms don't apply to industrial VFD troubleshooting.

Scenario D: The VFD Shows an Error or Alarm in Auto Mode — Check the Fault Log

This is actually the easiest to diagnose, if you stop and look. The VFD display shows a fault code. Maybe it's "Alarm 4" (Mains phase loss). Maybe it's "Alarm 14" (Ground fault). Maybe it's "Warning 59" (Current limit). But instead of reading the manual or checking the display, people panic and call the distributor. (I've been there. After the third late delivery from a vendor, I was ready to give up on them entirely. What finally helped was building in buffer time rather than trusting their estimates. But I digress.)

The fix:

• Write down the fault code. It's displayed right there on the VFD's control panel.
• Look it up in the Danfoss VFD manual for your specific model. Or Google it. (Honestly, this is 90% of the fix.)
• Check the Danfoss VFD parameter list for the specific warning or alarm. The manual is surprisingly easy to navigate, once you know the parameter number.

I've kept a folder of common Danfoss VFD fault codes and fixes on my desktop for the last 3 years. It's saved us thousands in emergency tech support calls. Most faults are caused by wiring issues or electrical supply problems — not the VFD itself.

How to Quickly Determine Which Scenario You're In

Here's a quick decision tree, based on what I've learned from managing hundreds of VFD-related issues across our facility:

1. Does the display show anything? No → Check incoming power. Yes → Go to step 2.
2. Does it show a fault code? Yes → Look up the fault code in the manual (Scenario D). No → Go to step 3.
3. Does it switch to Auto mode at all? No → Can't change anything → Check the password (Scenario C). Yes → Go to step 4.
4. Does the motor start and run normally, but only in Manual mode? Yes → Control signal issue (Scenario A). No → Erratic behavior → Terminal configuration or parameter issue (Scenario B).

After 5 years of managing procurement, I've come to believe that 80% of VFD issues are simple — loose wiring, wrong parameters, forgotten passwords. The other 20% require a factory call or replacement. But even those are manageable if you've done your basic troubleshooting first.

That "save $80 by not calling a technician" choice? I've made it. And I've regretted it when the downtime cost $2,000. Sometimes spending the money on a qualified technician or a genuine Danfoss part is the right call. Other times, a $2 fuse is all you need. The key is knowing which scenario you're in.