A Danfoss VLT HVAC Drive FC-102 tells you two different things with its two LEDs, and mixing them up costs you time. A steady yellow light is a warning: the drive saw something it did not like but the supply fan or the chilled-water pump is still turning. A flashing red light is an alarm: the drive tripped and dropped the load to protect itself, the motor, or the cable. Next to either light is a number, and that number points at one specific protection function. Read it before you touch anything. The two mistakes we clean up most often in Central Florida mechanical rooms are the same on any drive line: condemning a healthy drive and swapping it (expensive, usually wrong), and hitting reset on a fault that is doing its job until the motor finally lets go (cheap, and it ends a fan motor early). This is a code-by-code field guide to the FC-102 faults that actually show up on HVAC fans and pumps, with every meaning traced to Danfoss's VLT HVAC Drive FC 102 Programming Guide. If a code is not in Danfoss's warning and alarm list, it is not in this guide.

If you need the why behind these drives being on the fans and pumps at all, the motor-control and energy fundamentals, start with our pillar explainer, Variable Frequency Drives in the HVAC Market. And if you also run other drive lines, the companion pieces walk the same first-checks discipline: ABB ACH580 Fault Codes on HVAC Fans and Pumps and Yaskawa Z1000 Fault Codes on HVAC Fans and Pumps. This post assumes you know what a VFD does and need to get an FC-102 running again.

How the FC-102 reports a fault

The distinction between warning and alarm is built into the drive. A warning stays active only as long as its cause is present, and the motor can keep running; an alarm trips the drive, and you reset it once the cause is fixed. Some faults can be either: whether the drive warns first or trips outright depends on how a parameter is set. Motor thermal protection (parameter 1-90) is the classic example, where you choose whether an overheating motor produces a warning or an alarm. On a BACnet MS/TP or Modbus job, the same number you read on the local panel is what lands in the BAS, so your controls contractor and your service tech are looking at the same alarm.

Two behaviors are worth knowing cold before you start resetting. First, some alarms are trip-locked: the manual is explicit that mains power must be switched off before a trip-locked alarm can be cleared, which is a deliberate extra layer of protection, not a broken reset button. Second, the FC-102 ships with automatic reset enabled by default (parameter 14-20), and Danfoss flags the obvious hazard in its own text: automatic wake-up is possible. A fan that faults, auto-resets, and spins back up on its own is a real safety consideration when someone has a hand near a wheel. Confirm the reset mode on any unit you are working on.

Overcurrent and short circuit: 13 and 16

Alarm 13, Over current is the one you will see most on fans. Per the manual, it means the inverter peak current limit (approximately 200% of the rated current) is exceeded. Danfoss notes the warning lasts about 1.5 seconds and then the drive trips, and it names the usual culprits directly: shock loading or quick acceleration with high-inertia loads can cause this fault. That description fits a plenum fan wheel with a lot of rotating mass exactly. First checks, in the order the manual lays them out:

• Turn the shaft. Danfoss's first troubleshooting step is to remove power and check if the motor shaft can be turned. On a fan, a seized bearing, a wheel packed with cottonwood, or a backdraft damper stuck closed will spike current on start. On a pump, a closed isolation valve or an air-bound casing does the same. Lock out and bar it over by hand.
• Match the drive to the motor. The manual says to check that the motor size matches the frequency converter. A drive dropped a frame size to save money trips on any real load.
• Confirm the motor data. Danfoss points to parameters 1-20 through 1-25 (the nameplate set). Wrong motor data throws off the drive's current model.
• Lengthen the ramp. Because quick acceleration into a high-inertia wheel is a named cause, a gentler acceleration time is often the fix on a big fan that only trips 13 on start.

Alarm 16, Short circuit is the harder cousin and the manual keeps it short: there is short-circuiting in the motor or motor wiring. This is not a reset-and-run condition. Danfoss's only troubleshooting line is to remove power and repair the short. Lock out, disconnect the motor leads at the drive, and megger the cable and the windings before you re-energize anything.

Ground faults: 14 and 45

Alarm 14, Earth (ground) fault is the code that finds moisture and insulation breakdown, which on roof-mounted AHU and cooling-tower fans in Florida is a matter of when, not if. The manual explains the mechanism plainly: there is current from the output phase-to-ground, either in the cable between the frequency converter and the motor, or in the motor itself. The drive compares current leaving against current returning, and trips when the deviation is too large. First checks per Danfoss: remove power and repair the ground fault, then check for ground faults in the motor by measuring the resistance to ground of the motor cables and the motor with a megohmmeter. The manual adds a step techs miss: after replacing a power card, reset the offset in the three current transducers by running a manual initialization or a full AMA, because a stale offset can produce a false 14. Alarm 45, Earth fault 2 is the related ground fault the manual ties to proper grounding, correct wire size, and cable short circuits or leakage. Do not defeat a ground fault to keep a fan running: a wet motor to ground is a shock and fire path.

Mains and DC bus: 4, 7, and 8

These three tell you about power quality upstream of the drive, which matters in a climate where summer afternoon storms and utility switching hammer the grid.

• Warning/Alarm 4, Mains phase loss. Danfoss: a phase is missing on the supply side, or the mains voltage imbalance is too high. This message also appears for a fault in the input rectifier. The troubleshooting step is to check the supply voltage and supply currents to the drive. A corroded lug dropping one incoming phase is the usual Florida cause; the response to imbalance is set in parameter 14-12.
• Warning/Alarm 7, DC overvoltage. Per the manual, if the DC-link voltage exceeds the limit, the drive trips after a certain time. On a high-inertia fan decelerating fast, the motor pumps energy back into the bus and pushes it over. Danfoss's fixes, in order: extend the ramp time, change the ramp type, activate the brake function (parameter 2-10), or connect a brake resistor. If the fan must stop quickly, the answer is a brake resistor, not just a longer ramp.
• Warning/Alarm 8, DC under voltage. The manual: if the DC-link voltage drops below the undervoltage limit, the drive checks for a 24 V DC back-up and, finding none, trips after a fixed delay. Danfoss says to check that the supply voltage matches the drive voltage and to run an input voltage test. Same hunt as a dropped phase: incoming voltage, fuses, and the disconnect.

Motor phase missing: 30, 31, 32

The FC-102 watches its own output. Alarm 30, Motor phase U missing, Alarm 31, Motor phase V missing, and Alarm 32, Motor phase W missing each mean exactly what they say per the manual: that phase, between the drive and the motor, is gone. Danfoss's troubleshooting for all three is the same, and it carries a high-voltage caution: remove power from the drive and check the named motor phase. These show up after a contactor swap, a bypass change, or a lug that vibrated loose on a rooftop unit, not on a system that ran fine for years. If you get one right after panel work, suspect the connection someone just touched.

Drive over-temperature: 29, 65, 69, and the fan faults 23 and 24

Florida runs these drives hot, and the FC-102 splits over-temperature by which board is cooking. Reading the number tells you which enclosure to open.

• Alarm 29, Heat sink temp. The maximum temperature of the heat sink is exceeded. The manual's checklist is the one to work: ambient temperature too high, motor cables too long, incorrect airflow clearance above and below the drive, blocked airflow around the drive, a damaged heat sink fan, or a dirty heat sink. In a Florida mechanical room that reads 105 degrees F in August, a dirty heat sink is the number-one cause. Blow it out. The manual notes the fault holds until the temperature drops below a defined reset point, so it will not clear the instant you open the door.
• Warning/Alarm 65, Control card over temperature. Danfoss puts the control-card cutout at 85 degrees C (185 degrees F). Checks: confirm ambient is within limits, check for clogged filters, check fan operation, check the control card.
• Alarm 69, Power card temperature. The temperature sensor on the power card is either too hot or too cold. The manual's checks mirror the others: ambient within limits, clogged filters, fan operation, then the power card itself. The too-cold half of that definition is real but rare in Florida.
• Warning 23, Internal fan fault and Warning 24, External fan fault. These are the drive's own cooling fans. The manual describes the monitor as a protective function that checks whether the fan is running or mounted, and says to check for proper fan operation and to cycle power and watch the fan run briefly at start-up. If a heat-sink fan has quit, 23 or 24 is often the code that shows before 29 does.

Motor protection: 9, 10, 11, and 12

These are the codes most tempting to reset away, and the ones most likely to be saving a motor.

• Warning/Alarm 9, Inverter overload. Per the manual, the drive has run with more than 100% overload for too long and is about to cut out. The electronic thermal counter warns at 98% and trips at 100%, and Danfoss is specific that the drive cannot be reset until the counter is below 90%, so a nuisance reset will not take. Compare the output current on the display to the drive rating and to the measured motor current before you do anything else.
• Warning/Alarm 10, Motor overload temperature. According to the electronic thermal protection (ETR), the motor is too hot. Danfoss says to check for motor overheating, check whether the motor is mechanically overloaded, verify the motor current in parameter 1-24, and confirm the nameplate data in parameters 1-20 to 1-25. On a belt-drive fan held at low speed for long stretches, this is frequently a real condition, not a glitch (see the affinity-law note below).
• Warning/Alarm 11, Motor thermistor overtemp. This one comes from a thermistor or PTC wired to the drive. The manual's first move is to check whether the thermistor is disconnected, then check for motor overheating and mechanical overload, and confirm the thermistor is landed correctly and that parameter 1-93 points at the right terminal. A broken thermistor lead reads as an open and can trip 11 on a perfectly cool motor.
• Warning/Alarm 12, Torque limit. Per the manual, the torque has exceeded the value in parameter 4-16 Torque Limit Motor Mode (or 4-17 in generator mode). Danfoss's guidance: if the limit is hit during ramp-up, extend the ramp; if while running, raise the limit only after confirming the system can operate safely at higher torque; and check the application for excessive current draw. On a pump, 12 while running often means a failed check valve or a system curve that moved.

Comms and safety: 17, 2, and 68

These do not mean the drive is broken. They mean it lost a signal it needs, or a safety circuit did its job.

• Warning/Alarm 17, Control word timeout. There is no communication to the frequency converter. The manual says to check the serial communication cable connections, increase the control timeout time (parameter 8-03), verify the communication equipment is working, and confirm proper EMC installation. On a fieldbus-driven AHU, this is a lost BAS command, not a drive defect.
• Warning/Alarm 2, Live zero error. This appears when an analog input drops below 50% of its programmed minimum, and the manual names the causes directly: broken wiring or a faulty device sending the signal. On a 4 to 20 mA pressure or flow input, a 2 usually means the sensor or its wiring failed, not the drive. Check the analog terminals (53, 54, and common 55) and confirm the switch and programming match the signal type.
• Alarm 68, Safe Stop activated. Safe Torque Off has opened. Per the manual, to resume normal operation you apply 24 V DC to terminal 37 and then send a reset. This is a safety function working: check the device that opened the STO circuit (a firestat, a smoke-detector relay, an E-stop) before you assume a drive problem.

Why the low-speed motor faults matter: the affinity laws

Several checks above, a motor overheating at low speed, a thermal counter creeping up, trace back to how these drives save energy in the first place. Fan and pump power follows the cube of speed. The U.S. Department of Energy's guide Variable Speed Pumping: A Guide to Successful Applications states that the power a pump draws varies with the cube of its speed, which is why matching speed to demand produces such large savings. Slow a fan to 80% speed and shaft power falls to roughly 0.8 cubed, about 51%, so you get four-fifths of the airflow for about half the draw. That cube law is the whole economic case for putting an FC-102 on a constant-volume AHU in a Florida climate that runs 4,000-plus cooling hours a year. But the same physics means the drive spends much of its life at reduced speed, where a self-cooled TEFC motor gets less airflow over its own frame. That is exactly why motor-side codes like 10 and 9 deserve a real look instead of a reset: the drive is often reporting the predictable downside of the very thing that saves the energy.

A reset discipline for the field

One rule keeps you out of trouble on an FC-102: read the number, decide whether it is protecting something physical, and only then reset. Overcurrent (13), short circuit (16), ground fault (14 and 45), inverter overload (9), motor overtemp (10), and torque limit (12) all point at a real condition; find it first. The mains, DC-bus, over-temperature, and comms codes are usually a wiring or settings hunt you can work methodically. Log the code, the date, and what you found. On a fleet of rooftop units the pattern in those logs, every 13 on the same fan every August, tells you more than any single trip. Remember that automatic reset is on by default and can wake a fan on its own, and that trip-locked alarms need mains cycled before they clear. If a fault re-trips after one confirming reset and the mechanical and electrical checks come back clean, that is the point to escalate to a drive-level diagnosis, not before.

Serving Orlando and Central Florida, we supply, program, and service Danfoss VLT HVAC Drive FC-102 units on commercial AHUs, cooling towers, and chilled- and condenser-water pumps. If a code in this list is re-tripping on your equipment, we can walk the first checks with your team or come turn a wrench.