# Yaskawa VFD fault codes: oC, ov, GF and what to check

Yaskawa A1000 fault codes explained: what oC, ov, GF, LF, Uv1, PF, oH, oH1, oL1 and oL2 mean, how to tell a fault from an alarm, which U2 and U3 records to copy before you reset, and what the manual says to check first. Every point cites a printed page of Yaskawa's A1000 Technical Manual.

**Short answer:** On a Yaskawa A1000, a code with a steady ALM light is a fault and the drive has stopped. A blinking ALM light with a flashing code is only an alarm. The common trips are oC (overcurrent), ov (DC bus overvoltage), GF (ground fault), oH and oH1 (heatsink overheat), oL1 and oL2 (motor and drive overload), Uv1 (DC bus undervoltage), PF (input phase loss) and LF (output phase loss). Before you reset, copy U2-01 to U2-20 and U3-01 to U3-10.

**Applies to:** Yaskawa A1000 drives. Other Yaskawa series, such as V1000, GA500, GA800 and U1000, have their own manuals, and their codes and parameters can differ, so check yours · **Manual used:** Yaskawa AC Drive A1000 Technical Manual, SIEP C710616 27C (July 2009). Page numbers below are the printed pages of that edition. Later editions, including the one for US (CIMR-AU) models, are paginated differently, so match by code name, not page · **Last verified:** 1 Nov 2026 · **Author:** Muhammed Fahiz

## What does this look like on site?

A cooling tower fan, a compressor or a pump on an A1000 stops. The keypad shows oC, ov or GF and the ALM light is on. Someone presses RESET, the drive runs for a while, then trips again.

Sometimes there's no keypad at all. On Electrician Talk in October 2024, Mike in Canada described an A1000 inside an Atlas Copco air compressor that was tripping on overcurrent. The drive had no keypad, so he was reading the fault from the compressor's own controller, and "with the motor disconnected the drive still faults out immediately when you try to run it" ([thread](https://www.electriciantalk.com/threads/fixing-a-yaskawa-a1000-with-overcurrent-problem-just-getting-started.302269/)). We come back to what he found below.

This page covers what the common A1000 codes mean, what Yaskawa's own manual says to check, and the page where it says so, so you can confirm it in your copy. It doesn't reproduce Yaskawa's tables.

## How do you read a Yaskawa A1000 fault code?

First, decide whether it's a fault or an alarm. When the drive detects a fault, the code appears, the ALM light stays lit until the fault is reset, the output shuts off and the motor coasts to a stop. The fault contact switches too: MA-MC closes and MB-MC opens (p. 300). If the ALM light is blinking and the code is flashing, it's an alarm, and the drive may still be running (p. 301). Some codes, ov and oH among them, can appear either way, so the light matters (pp. 302-303).

| Code | Name in the manual | What it means | Manual page (27C) |
|---|---|---|---|
| oC | Overcurrent | Output current went above the overcurrent level | p. 311 |
| GF | Ground Fault | Current to ground on the output side went above 50% of the drive's rated current | p. 310 |
| LF / LF2 | Output Phase Loss / Output current imbalance | A motor phase is open or the phase currents are uneven | p. 310 |
| ov | Overvoltage | DC bus above about 410 V (200 V class) or 820 V (400 V class) | p. 315 |
| Uv1 | DC Bus Undervoltage | DC bus below the L2-05 level, about 190 V or 380 V | p. 317 |
| Uv2 / Uv3 | Control power undervoltage / soft-charge circuit fault | Control supply or soft-charge bypass problem | p. 318 |
| PF | Input Phase Loss | Open supply phase or large imbalance between phases | p. 316 |
| oH / oH1 | Heatsink Overheat / Overheat 1 | Heatsink above the pre-alarm level (oH) or the overheat level (oH1) | p. 313 |
| oL1 / oL2 | Motor Overload / Drive Overload | Electronic motor overload, or the drive's thermal protection | p. 314 |
| oL3 | Overtorque Detection 1 | Current above L6-02 for longer than L6-03 | p. 314 |
| FAn | Internal fan fault | Internal fan or, on some large models, the magnetic contactor | p. 309 |

*Our summary of section 6.4 of the A1000 manual (27C). The full fault table runs from p. 301 to p. 319; the alarms follow from p. 302.*

## What should you write down before you press RESET?

Write down the fault records first. The manual's diagnosis procedure starts with the U2 monitors, which show the drive's state just before the fault (p. 334).

- **The fault trace (U2):** U2-01 is the current fault and U2-02 the previous one. U2-03 to U2-20 hold the values at that fault, including output current (U2-05), output voltage (U2-07), DC bus voltage (U2-08), the input terminal states (U2-11), cumulative run time (U2-14) and heatsink temperature (U2-20) (pp. 449-450).
- **The last ten faults (U3):** U3-01 to U3-10 hold the ten most recent faults, newest in U3-01, and U3-11 onward hold the run hours at each one. Once ten faults are stored, the oldest is dropped (p. 450).
- **The wear monitors (U4):** U4-03 is cooling fan run time, U4-04 the fan's maintenance percentage, U4-05 the capacitors', U4-06 the soft-charge bypass relay's and U4-07 the IGBTs' (pp. 449-450). Several fault entries below send you to these numbers.

The current and bus voltage at the trip go in the job report with the code. Our post on [what to write before you close a breakdown job](https://hangersearch.com/blog/its-running-now-is-not-a-fix) covers the rest of that record.

There are three ways to reset an A1000: the RESET key, the fault reset input (terminal S4 by default, H1-04 = 14), or switching the power off and waiting until the display goes out before switching it back on. The drive ignores a reset while a Run command is present (p. 335). If you try anyway, you'll see CrST, "Cannot Reset" (p. 320).

## oC: what causes overcurrent on a Yaskawa A1000?

The manual lists more causes for oC than for most codes (p. 311). In its order, they are:

- the motor damaged by overheating, or its insulation damaged
- a shorted motor cable or a grounding problem
- a load that's too heavy, or sudden swings in current
- acceleration or deceleration times that are too short
- a special motor, or a motor larger than the drive is rated for
- an output contactor switching while the drive is putting out current
- V/f settings that don't suit the motor (E1-04 to E1-10)
- too much torque compensation (C4-01)
- electrical noise
- overexcitation deceleration gain set too high (n3-13)
- a Run command given while the motor is still coasting
- the wrong PM motor code (E5-01), or a control mode in A1-02 that doesn't match the motor type
- a drive whose rated output current is too small

The first two causes are both about insulation: check the motor's insulation resistance and the resistance from the motor cables to the ground terminal. For a coasting load such as a fan, the manual's fix for restarting into a spinning motor is speed search, either at start (b3-01 = 1) or from a terminal (H1 = 61 or 62) (p. 311).

The manual also notes that a mechanical overload can show up as oC or oL1, and suggests using torque detection to warn before either happens (p. 264).

### What does a field oC report look like?

Back to the compressor drive. With the motor disconnected the A1000 still tripped at once, which rules out the motor and cable. Later the same day, Mike in Canada posted that he'd tested the drive and found "a bad output transistor on L2 (shorted)", and it went to a repair shop (Electrician Talk, posts #3 and #4, 22-23 Oct 2024). The thread doesn't report the final repair. That's one report, not a confirmed fix, but it matches the manual: an output short with the motor disconnected points at the drive itself. Our post on [IGBT failure symptoms and the diode-mode test](https://hangersearch.com/blog/igbt-failure-symptoms-vfd) covers the terminal checks, and [isolating a VFD overcurrent trip](https://hangersearch.com/blog/vfd-overcurrent-fault-causes) covers splitting the motor, cable, load and drive in general.

## GF and LF: is it the motor, the cable or the drive?

**GF** trips when the current to ground on the output side goes above 50% of the drive's rated current. The listed causes are damaged motor insulation, a damaged motor cable, high leakage current at the drive output, a PM motor restarted while coasting, and a hardware fault in the drive. The fixes are to check the motor's insulation resistance and the resistance from the cable to ground, replace whichever is damaged, and for leakage, reduce the carrier frequency and the stray capacitance (p. 310). Ground fault detection is L8-09, which is enabled by default (p. 271).

**LF** (output phase loss) only trips if L8-07 is set to 1 or 2, and the default is 0, disabled (pp. 270-271). The causes are a disconnected output cable, a damaged motor winding, a loose output terminal, a motor whose rated current is under 5% of the drive's, a damaged output transistor, or a single-phase motor, which the drive can't run (p. 310). **LF2** (current imbalance, when L8-29 = 1) adds uneven motor phases: measure the line-to-line resistance of each phase and check they match (p. 310).

The A1000 manual doesn't give an insulation-test procedure. The usual practice is to disconnect the motor cable at the drive so the test voltage doesn't reach the drive's output, then test the motor and cable to ground. Before touching anything inside, wait at least the time marked on the drive, because the capacitors stay charged after the power is off (p. 21).

## ov: why does a Yaskawa A1000 trip on overvoltage?

ov means the DC bus went above about 410 V on a 200 V class drive or about 820 V on a 400 V class drive. The causes listed (p. 315):

- **Regeneration.** Deceleration is too short and energy is flowing back from the motor. The fixes are a longer deceleration time (C1-02, C1-04, C1-06, C1-08), a dynamic braking option, or stall prevention during deceleration (L3-04 = 1), which is on by default.
- **Overshoot on acceleration.** Fast acceleration pushes the motor past the speed reference. The fixes are a longer acceleration time, longer S-curves, or overvoltage suppression (L3-11 = 1).
- **Supply surges.** The fix is a DC reactor. The manual notes surges can come from a thyristor converter and a phase-advancing capacitor on the same supply. The alarm entry for ov adds AC reactors (p. 323).
- **A ground fault on the output** overcharging the bus capacitors. Check the motor wiring for ground faults.
- **Other causes:** speed search settings, a supply voltage that's too high, braking transistor wiring errors, PG encoder wiring or noise, wrong load inertia settings, and motor hunting.

One thing to check if a braking resistor is fitted: the manual says L3-04 = 1 shouldn't be used with a braking resistor, because stall prevention acts before the resistor gets a chance. Setting 3 is the one meant for a braking option (p. 258). So a resistor can be installed and still not doing the work.

![Ink line drawing of an incoming power chain on a plant wall: a fused disconnect with its door open showing three cartridge fuses, a three-phase line reactor on a wall bracket, and a drive cabinet with a blank keypad, joined by conduit along a cable tray](/blog/media/yaskawa-vfd-fault-codes-mvxcpi18.png)

## Uv1 and PF: is it the supply or the drive?

**Uv1** means the DC bus fell below the L2-05 level, about 190 V for the 200 V class and about 380 V for the 400 V class (350 V if E1-01 is below 400) (p. 317). With L2-01 at its default of 0, the drive trips Uv1 if power isn't back within 15 ms (p. 249). The causes are:

- a lost input phase or a loose input terminal
- a supply voltage problem (if the supply itself is fine, the manual says to check the main-circuit contactor)
- a power interruption
- worn main capacitors (replace if U4-05 is over 90%)
- a damaged soft-charge bypass relay (replace if U4-06 is over 90%) (p. 317)

The Uv alarm entry adds an undersized supply transformer and hot air inside the drive (p. 324). **Uv3** is a separate soft-charge circuit fault, and also points to U4-06 (p. 318).

**PF** (input phase loss) is on by default (L8-05 = 1). The drive detects it from the ripple on the DC bus, so it can trip on a supply imbalance or worn capacitors as well as an open phase (p. 270). The listed causes are a lost phase in the input wiring, loose input terminals, a fluctuating supply, poor balance between phases, and worn capacitors, again with U4-05 over 90% as the replacement point (p. 316).

So for both codes, check the supply side before the drive: fuses, the disconnect, input terminals and the voltage on each phase. Then read U4-05 and U4-06.

## oH, oH1, oL1 and oL2: heat and overload

- **oH** means the heatsink passed the overheat pre-alarm level set in L8-02, and **oH1** that it passed the drive's overheat level. Both list a high ambient temperature and a load that's too heavy. oH adds a stopped internal cooling fan (p. 313). The fixes are better circulation in the enclosure, a fan or air conditioner, less load, and a lower carrier frequency (C6-02). The oH alarm entry adds restricted airflow and dust or debris clogging the fan (p. 322). L8-03's default is 3, so at the pre-alarm level the drive warns and keeps running (p. 269).
- **Fans wear out.** The manual estimates 10 years for the cooling fans and the electrolytic capacitors, under stated conditions: a 40 deg C yearly average, 80% load and 24-hour running (p. 349). U4-04 shows how far the fan is through that life (p. 450). Before changing a fan, switch off and wait at least 15 minutes for the heatsink to cool (p. 21).

![Ink line drawing of an industrial air handling unit with an access panel removed, showing a centrifugal fan wheel, a belt-driven motor on a base, a dust-loaded filter bank and a small drive enclosure on the side](/blog/media/yaskawa-vfd-fault-codes-mvxcqnvq.png)

- **oL1** is the drive's electronic motor overload. The causes include a load that's too heavy, ramps that are too short, a general-purpose motor run slowly under heavy load, V/f output voltage set too high, the wrong motor rated current in E2-01 (enter the nameplate value), several motors on one drive (set L1-01 = 0 and fit a thermal relay to each motor), overexcitation, speed search settings, and input phase loss (p. 314).
- **oL2** is the drive's own thermal protection. Its causes overlap with oL1, plus a drive that's too small and overload at low speed, where the fixes are less load, a drive one frame size larger, or a lower carrier frequency (p. 314).
- **oL3** is overtorque detection: current above L6-02 for longer than L6-03. The manual lists wrong settings or a problem on the machine side, such as a jammed machine (p. 314).

## What should you check, in order?

1. **Copy U2-01 to U2-20 and U3-01 to U3-10.** **Expect:** the code, the current and bus voltage at the trip, and whether it's happened before. **If not:** if U2-01 doesn't match the display, note both before anyone resets again. (pp. 334, 449-450)
2. **Check the ALM light.** **Expect:** steady means fault, blinking means alarm. **If not:** for an alarm, find the code in the alarm list; the drive may not have stopped. (pp. 300-303)
3. **For Uv1, PF or ov, check the supply side.** **Expect:** all three phases present and balanced, tight input terminals. **If not:** fix the supply first. (pp. 315-317)
4. **For oC, GF or LF, power down, wait the time marked on the drive, then test the motor and cable with the motor cable disconnected at the drive.** **Expect:** good insulation, equal phase resistances. **If not:** repair the motor or cable before powering up. (pp. 21, 310-311)
5. **If the fault returns with the motor disconnected,** suspect the drive's output stage. **Expect:** no trip with no load. **If not:** it's a drive repair or replacement. (p. 310; Electrician Talk, posts #1-#3)
6. **For ov with a braking resistor fitted, check L3-04.** **Expect:** a setting meant for a braking option. **If not:** correct it, then check the deceleration time. (pp. 258, 315)
7. **For oH, oH1 or FAn, check the fan, airflow and ambient temperature, and read U4-04.** **Expect:** fan running, fins clear. **If not:** clean or replace, then reset the fan maintenance count. (pp. 313, 322, 349)
8. **Only then change ramps, V/f, torque compensation or speed search settings.** **Expect:** settings that suit the load. **If not:** change one at a time and write down the old value. (pp. 311, 314-315)
9. **If U4-05 or U4-06 are over 90%,** the manual's fix is replacing the capacitors or soft-charge relay, or the board or drive. (pp. 316-317)

## What does the manual say vs what has fixed it in the field?

| | Manual cause list | Field fixes reported |
|---|---|---|
| oC | Motor or cable insulation, load, ramps, motor size, output contactor, V/f, torque compensation, noise, speed search, PM settings, drive too small (p. 311) | One report: an A1000 in an air compressor kept tripping with the motor disconnected; testing found a shorted output transistor on L2 (Mike in Canada, Electrician Talk, Oct 2024). Final repair not reported |
| ov | Regeneration, overshoot, braking load, supply surge, output ground fault, speed search, high supply, braking wiring, PG, noise (p. 315) | No field reports found yet. |
| GF | Motor insulation, cable, leakage current, PM restart, hardware (p. 310) | No field reports found yet. |
| Uv1 / PF | Phase loss, loose terminals, supply, worn capacitors, soft-charge relay (pp. 316-317) | No field reports found yet. |
| Source | Yaskawa A1000 Technical Manual, SIEP C710616 27C | Electrician Talk thread, Oct 2024 |
| Confidence | Official | Single report, outcome not confirmed |

We looked for forum threads where someone fixed a specific A1000 code and said it held. The compressor thread was the only one we could cite, and it stops at the diagnosis. If you have a confirmed fix, it's worth writing it up with the code, the U2 records and what you replaced.

## Fault-code entry (summary block)

| Field | Value |
|---|---|
| Codes | oC, GF, LF, LF2, ov, Uv1, Uv2, Uv3, PF, oH, oH1, oL1, oL2, oL3, FAn |
| Brand / series | Yaskawa / A1000 |
| Official names | See the table under "How do you read a Yaskawa A1000 fault code?" |
| Fault or alarm | Steady ALM = fault, drive stopped. Blinking ALM and flashing code = alarm |
| Likely causes (ranked) | oC: motor or cable insulation, load, ramps, then settings and drive hardware. ov: regeneration, overshoot, supply surge, then settings. GF: motor, cable, leakage. Uv1/PF: supply, terminals, then worn capacitors or soft-charge relay |
| Checks in order | 1. Copy U2 and U3. 2. Fault or alarm. 3. Supply (Uv1, PF, ov). 4. Motor and cable disconnected at the drive (oC, GF, LF). 5. Fault with motor off = drive. 6. L3-04 if a resistor is fitted. 7. Fan and airflow. 8. Settings. 9. U4-05/U4-06 over 90% |
| Field fixes reported | oC: one report, shorted output transistor (not confirmed fixed). Others: No field reports found yet |
| Related codes | CrST (reset with Run command present), LT-1 and LT-2 maintenance alarms |
| Manual reference | Yaskawa A1000 Technical Manual SIEP C710616 27C, pp. 21, 249, 258, 264, 269-271, 300-303, 309-318, 320, 322-324, 334-335, 349, 449-450 |
| Last verified | 1 Nov 2026 |

## Why does the same Yaskawa code keep coming back after a reset?

Usually because the reset clears the trip, not the cause. If auto restart has been set up, the drive may already have reset itself before it stayed in the fault. L5-01 sets the number of attempts, from 0 to 10, and the default is 0. The count goes back to zero after ten minutes of normal running, a manual reset or a power cycle (p. 264). Read U3-01 to U3-10 to see whether it's the same code each time and how many run hours apart.

If this drive has tripped on the same code before, someone may already have found the cause. Our guide on [finding a past service fix for a repeat machine fault](https://hangersearch.com/blog/find-past-service-fix-repeat-machine-fault) covers where to look, and [same machine, five descriptions](https://hangersearch.com/blog/repeat-breakdown-same-machine) covers why repeat faults are hard to connect when each one was written up differently. For other brands, see our pages on [Delta VFD fault codes](https://hangersearch.com/blog/delta-vfd-fault-codes), [Inovance MD500 fault codes](https://hangersearch.com/blog/inovance-fault-codes) and [INVT GD20 fault codes](https://hangersearch.com/blog/invt-vfd-fault-codes).

## How would you find this in your own archive?

If your team has cleared oC or ov on this drive before, the cause is probably in a service report somewhere. HangerSearch searches your own finalized reports where they already are, shows the few prior cases that match and why each matched, and opens the cited page. Nothing is generated, and if there's no match it says so. See [how we handle your reports](https://hangersearch.com/trust).

Book a 20-minute demo: hangersearch.com

## FAQ

### What does oC mean on a Yaskawa drive?
On an A1000, oC is overcurrent: the output current went above the drive's overcurrent level. The manual's causes start with motor insulation and shorted cables, then load, ramp times, motor size and settings (A1000 Technical Manual 27C, p. 311).

### What is the difference between a fault and an alarm on a Yaskawa A1000?
A fault stops the drive. The ALM light stays on, the motor coasts to a stop and the fault contact switches. An alarm shows a flashing code with a blinking ALM light and doesn't necessarily stop the drive (pp. 300-301).

### How do I fix ov on a Yaskawa A1000?
ov is DC bus overvoltage, usually energy coming back from the load as it slows. Lengthen the deceleration time or add a dynamic braking option. If a braking resistor is fitted, check that L3-04 isn't set to 1, which the manual says not to use with a resistor (pp. 258, 315).

### Where is the fault history on a Yaskawa A1000?
U3-01 to U3-10 hold the last ten faults, with run hours at each in U3-11 onward. U2-01 and U2-02 are the current and previous fault, and U2-03 to U2-20 hold the current, voltage and other values at the fault (pp. 449-450).

### Why won't my Yaskawa drive reset?
The A1000 ignores a reset while a Run command is present, and shows CrST if you try. Remove the Run command first, then press RESET or use the reset input (pp. 320, 335).

## Sources

1. Yaskawa Electric, "YASKAWA AC Drive A1000 Technical Manual", SIEP C710616 27C (July 2009, 544 pages). Public copy: https://www.herott.cz/fotky21314/Y_A1000_TM_EN.pdf. Read 1 Nov 2026. Supports: safety and capacitor discharge (p. 21); L2-01 (p. 249); L3-04 (p. 258); L5-01 and the overtorque note (p. 264); L8-02/L8-03 (p. 269); L8-05, L8-07 (pp. 270-271); L8-09 (p. 271); fault response and fault vs alarm display (pp. 300-303); FAn (p. 309); GF, LF, LF2 (p. 310); oC (p. 311); oH, oH1 (p. 313); oL1, oL2, oL3 (p. 314); ov (p. 315); PF (p. 316); Uv1 (p. 317); Uv3 (p. 318); CrST (p. 320); oH, ov and Uv alarms (pp. 322-324); diagnosing and resetting faults (pp. 334-335); performance life (p. 349); U2, U3, U4 monitors (pp. 449-450). Printed page numbers. Paraphrased.
2. Mike in Canada, "Fixing a Yaskawa A1000 with overcurrent problem - just getting started", Electrician Talk, posts #1, #3 and #4, 22-23 Oct 2024: https://www.electriciantalk.com/threads/fixing-a-yaskawa-a1000-with-overcurrent-problem-just-getting-started.302269/. Read 1 Nov 2026. Supports: the compressor drive report and the quotes "with the motor disconnected the drive still faults out immediately when you try to run it" and "a bad output transistor on L2 (shorted)".