Key takeaways
This guide covers the most common fault codes on Yaskawa variable frequency drives, following A1000 and GA800 family conventions: what each code means, likely causes in order of probability, and the first checks to run. It is written for maintenance technicians and plant engineers standing in front of a tripped drive. Exact code sets and trip levels vary by series and firmware, so confirm details in the technical manual for your specific model.
When a Yaskawa drive trips, it records a fault trace in the U2 monitor group: output frequency, output current, output voltage, and DC bus voltage captured at the instant of the fault. The U3 fault history keeps the last several faults with cumulative operation time, so you can see whether this is a first event or a repeat offender.
The code tells you what tripped; the trace tells you why. An OC with current pegged at a couple of hertz points to a jam or a short at start. An OV with high bus voltage during ramp-down points to regeneration. Write the trace values down before resetting, and compare them across events.
Apply lockout/tagout before any work inside the enclosure. After input power is removed, the DC bus capacitors still hold a lethal charge: wait at least the discharge time marked on the drive (often five minutes or more), confirm the charge indicator is out, and verify the bus is discharged with a meter rated for the voltage. Only qualified electricians work inside the cabinet.
Remember the machine side too: hydraulic pressure, chain tension, and gravity-loaded axes are stored energy just like the bus capacitors. Never bypass interlocks or safety circuits to keep a line running, and never jumper out an external fault input.
| Code | What it means | Most likely cause | First check |
|---|---|---|---|
| OC | Output overcurrent | Short in motor or cable, or mechanical jam | Insulation test motor and cable; bar the shaft |
| GF | Ground fault on output | Motor insulation breakdown or damaged cable | Phase-to-ground insulation resistance test |
| OV | DC bus overvoltage | Deceleration too fast (regeneration) | Note when it trips; extend decel time |
| UV1 | DC bus undervoltage | Supply dip or loose input connection | Log incoming voltage; inspect input terminals |
| OL1 | Motor overload | Mechanical overload or wrong motor current parameter | Compare running amps to nameplate |
| OL2 | Drive overload | Drive undersized or aggressive acceleration | Check load versus drive rating and accel time |
| OH/OH1 | Heatsink overheat | Failed fan or clogged heatsink/filters | Verify fans spin; clean heatsink and filter mats |
| PF | Input phase loss | Blown fuse or loose input lug | Measure all three input phases |
| LF | Output phase loss | Broken output conductor or loose terminal | Inspect output wiring and any output contactor |
| CE | Communication error | Network cable, noise, or master stopped polling | Check comms cable, termination, PLC status |
| EF | External fault via terminal | Machine device wired to a digital input | Identify the terminal and the device on it |
OC trips when output current exceeds roughly 200 percent of the drive rating. Causes in order of probability: a phase-to-phase or phase-to-ground short in the motor cable or windings, a mechanical jam or seized load, acceleration time set too short, incorrect motor data or V/f settings, and a contactor switching on the drive output under load.
First checks: disconnect the motor leads at the drive and run an insulation resistance test on cable and motor, then bar the shaft by hand to feel for binding. The trace tells you a lot here: OC at near-zero frequency is a short or a jam, OC at speed is more often load or parameter related. Our guide to troubleshooting VFD overcurrent faults walks the full sequence. If the driven axis feels rough or notchy when barred over, mechanical degradation may be the real problem; the same signs show up in failing servo motors.
GF means the drive detected leakage current to ground on the output. The usual suspects are aged or moisture-soaked motor insulation, a chafed cable, or very long motor leads. Run a phase-to-ground insulation test and inspect the cable run before condemning the drive.
OV means the DC bus exceeded its trip level (approximately 410 VDC on 200 V class and 820 VDC on 400 V class drives, model dependent). If it trips during deceleration, the motor is acting as a generator and pumping energy back into the bus. Extend the decel time, enable stall prevention during decel, or add a properly sized braking resistor. If a resistor is already fitted, verify it and its chopper actually work. OV at random times points instead to line surges or an overhauling load.
UV1 means the bus dropped below the undervoltage level. Causes: supply dips when large loads start elsewhere, loose or corroded input connections, worn input contactor tips, a blown input fuse, or a lost phase. First checks: torque-check the input terminals (power removed and bus verified dead), inspect the contactor, and log incoming voltage over a shift to catch dips you cannot see with a handheld meter.
OL1 is motor overload, calculated by the drive's electronic thermal model from the programmed motor rated current. Likely causes: genuine mechanical overload, a wrong motor current parameter, or a self-cooled motor running at low speed for long periods. Compare running amps against the motor nameplate first. The diagnostic logic mirrors a tripping bimetal relay, covered in our guide to motor overload relay trips.
OL2 is the drive itself exceeding its rating: an undersized drive, too-short acceleration ramps, high carrier frequency, or high ambient temperature all reduce headroom.
OH and OH1 are heatsink overtemperature (warning and trip levels). This is a cooling problem until proven otherwise: confirm the cooling fans spin, clean the heatsink fins, replace clogged cabinet filter mats, and check the enclosure ambient temperature against the drive specification.
PF flags excessive DC bus ripple from a missing or unbalanced input phase: check fuses, lugs, and upstream switchgear, and measure all three phases at the drive input. LF is the mirror image on the output side: a broken conductor, loose terminal, open motor winding, or an output contactor that opened under power.
CE means the drive stopped receiving valid control data over its fieldbus within the configured timeout. Check the cable and connectors, termination, grounding and noise, and whether the PLC or master actually stopped polling.
EF faults (EF0 through the numbered terminal faults) are different: a device wired to a multi-function digital input told the drive to fault. The number identifies the terminal, and the cause is outside the drive: an overtemperature switch, pressure switch, or the machine controller. Find the device on that terminal and fix it. Bypassing the input hides a real machine problem and may defeat a protective function.
A drive that trips OV once a shift is not an operator problem, it is an engineering backlog item hiding in plain sight. Log every fault as a downtime event with a cause code, keep the U2 trace values with the record, and track MTBF and MTTR per asset. When the data shows the same code recurring on the same drive, you have the business case for a braking resistor, a supply fix, or a motor rewind instead of endless resets.
These trips also erode availability, which is usually the biggest lever in OEE for manufacturing. A two-minute reset repeated ten times a week is over an hour of lost production that rarely appears in manual logs.
Fabrico is computer-vision-verified OEE plus closed-loop maintenance execution: cameras catch the stops and micro-stops that manual logs and sensors miss, and maintenance work orders close the loop from detection to fix. That means a recurring drive fault shows up as hard downtime data with a work order trail, not a note someone forgot to write. To see it on your own lines, book a Fabrico demo.
OC is an output overcurrent trip, typically at around 200 percent of the drive's rated current. The most common causes are a short in the motor or cable, a mechanical jam, or acceleration ramps set too short. Insulation-test the motor circuit and bar the shaft before restarting.
Use the keypad to open the U2 monitors for the fault trace (frequency, current, voltage, and DC bus voltage at the moment of trip) and the U3 monitors for the fault history. Menu navigation differs slightly by series, so follow the technical manual for your model.
During deceleration the motor regenerates energy into the DC bus, and if it cannot be absorbed fast enough the bus voltage exceeds the trip level. Extend the deceleration time, enable stall prevention during decel, or fit and verify a braking resistor.
UV1 means the DC bus voltage dropped too low, usually from a sagging or interrupted supply: brownouts, large loads starting on the same feeder, loose input connections, worn contactor tips, or a blown fuse. Logging the incoming voltage over time is the fastest way to confirm.
Only after you understand the cause. Repeated resets into a shorted motor or jammed load can damage the drive and the machine, and resetting an EF fault ignores a protective device that tripped for a reason. Read the fault trace, run the first checks, then reset.
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