Danfoss VFD Not Working in Auto Mode? A 6-Step Field Checklist for Engineers

When to Use This Checklist

You're on site. The system was running fine yesterday. Today, the Danfoss VFD refuses to start in Auto mode. It works in Hand mode—you can jog the motor via the LCP—but as soon as you switch to Auto, nothing happens.

This checklist is for that exact situation. It's also useful if you're commissioning a new installation and the drive won't respond to remote start commands. Six steps, no fluff. Here's what to check, in order.

Step 1: Verify the Power Supply to the Control Terminals

Most Danfoss VFDs (VLT® Micro Drive, FC 51, FC 102, etc.) require a 24 V DC external supply for the digital inputs when operating in Auto mode. The drive's internal 24 V supply often powers only the LCP and basic logic—not the fieldbus or external start/stop terminals.

The common mistake: Engineers assume that because the drive powers up and displays no alarms, the control supply is fine. But a partially tripped external supply (e.g., 19 V instead of 24 V) can cause erratic behavior in Auto mode while Hand mode still works.

Check the voltage between terminal 12 (+) and terminal 39 (common). If it's below 21.6 V DC (the minimum for reliable logic HIGH), trace back to the supply. We had a job last year where a loose screw on a terminal block gave us 22.1 V—intermittent. The drive would start in Auto maybe three out of five times. Tightened the connection, problem gone.

Step 2: Confirm Terminal 27 (Coast/Inverse) Is High

On Danfoss VLT® drives, terminal 27 is configured by default as "Coast inverse" (parameter 5-12). This means the drive will only run when terminal 27 is HIGH (24 V applied). If it's LOW (0 V), the drive coasts to stop—no matter what the start command says.

Most buyers focus on the start/stop wiring and completely miss this one. The question everyone asks is "Is the start signal present?" The question they should ask is "Is the coast inhibit signal present?"

Put your meter on terminal 27 and terminal 39. If you see less than 20 V, that's your problem. The fix is simple: either wire a jumper from terminal 12 to terminal 27, or configure parameter 5-12 to "No function" if you want the drive to start without an external enable signal.

Trick learned from a Danfoss application note: When troubleshooting "no Auto start" on an FC 102, always check terminal 27 first. It's the number one cause I've seen in the field—and it's often overlooked because it's not a traditional "start" wire.

Step 3: Verify the Reference Source

Auto mode on a Danfoss VFD typically means the frequency reference comes from an external source—either an analog input (terminal 53 or 54, 0–10 V or 4–20 mA) or a fieldbus command. If the drive is configured for an analog reference but the signal isn't there, it will sit at 0 RPM with no error.

Check parameter 3-15 (Reference Source). Is it set to "Analog input 53" or "Fieldbus"? Then confirm the actual signal on the relevant terminal. For a 4–20 mA signal, a reading of 0 mA usually means a broken wire or a disconnected loop. (Yes, that happened to me once—forgot to connect the shield back after a panel modification.)

Step 4: Check the Fieldbus Wiring (If Applicable)

If your Auto reference comes via Modbus RTU, BACnet, or Profibus, verify the physical layer. A loose termination resistor or a missing bias resistor can cause intermittent communication that the drive doesn't report as an alarm—it just doesn't respond to the reference.

For RS-485 (Modbus RTU): check that the two wires are not swapped (A/B polarity), and that termination resistors (120 Ω) are fitted only at the two ends of the bus. I've seen systems where every device had termination enabled—causing signal reflection and random dropouts.

Step 5: Review the Start Command Configuration (Parameter 5-10)

This is the one most people get right, but it's worth verifying. Parameter 5-10 defines how the drive receives its start command. Common settings:

• [0] Digital input – Start via a physical terminal (e.g., terminal 18 for start, terminal 19 for stop/reverse).
• [1] Fieldbus – Start via communication bus.
• [2] Auto/manual via LCP – The LCP Hand/Auto button controls the mode, but the start command source is still digital or fieldbus.

If parameter 5-10 is set to "Digital input" and you're trying to start via fieldbus, the drive will ignore your command. Conversely, if it's set to "Fieldbus" and your PLC program isn't sending the start bit, the drive won't run.