Fanuc servo alarm codes: find the module first, then read the digit

"The amp shows a 2." A lot of calls about a down Fanuc machine start exactly like that, and my first question never changes. Which amp? An α or αi cabinet usually has a power supply module, a spindle amp and one or more servo amplifier modules in one row, each with its own little STATUS display. A 2 on the power supply is a dead fan. Put that same 2 on a servo amp and it's low control voltage. Same digit, different fault.

So Fanuc servo alarm codes on the amplifier are only half an address. The other half is which module is talking and which generation it belongs to. Below is what the codes mean on each module, the CNC alarm number each one comes up as, and the order I check things in before anyone orders a part.

Who is who in the row

Left to right it's usually the PSM, then the SPM, then the SVMs. The PSM (power supply module) turns incoming AC into the DC link that feeds every module to its right. The SPM drives the spindle motor. An SVM (servo amplifier module) runs one, two or three axes. Fanuc calls them L, M and N, and their Pulsecoders plug into JF1, JF2 and JF3.

The part number prefix gives away the generation, and that decides which list of Fanuc servo alarm codes you should be reading.

Module	α series	αi series
PSM (power supply)	A06B-6077	A06B-6110
SVM (servo amplifier)	A06B-6079 (A06B-6080 on Series 20 and 21-GA)	A06B-6114
SPM (spindle amplifier)	A06B-6078	A06B-6111

Part number prefixes for the α and αi module families covered here.

β amplifiers are a separate product line with a code list of their own. I wouldn't read their display with any table here.

Know what normal looks like before you read anything as a fault, too. A dash on the PSM and a dash on an SVM are not the same message.

What you see	On a PSM	On an αi SVM
Display dark	No control power, or the control power circuit is bad	No power, a poor cable connection, or a blown F1 fuse / bad amplifier
Steady "-"	Not ready: MCC off, emergency stop state	Control power is up, waiting for the ready signal
"0"	Ready: MCC on, main circuit powered	Ready, motor powered
Digit with a dot	Warning: the PSM keeps running but will trip later	8., 9., A. are power stage alarms (see below)
Digit or letter, no dot	Alarm: the PSM will not run	Alarm
Blinking "-"	Not listed	Abnormal control power supply, feedback cable or Pulsecoder side

What the STATUS display shows on an αi PSM and SVM when nothing is wrong, and when something is.

 

Same digit, different module

This is the diagram I’d tape inside every cabinet door.

Where Fanuc servo alarm codes show up in an αi row, and what each STATUS display tells you before you read a digit.

Code	αi PSM	α PSM (shows 01 to 07)	αi SVM	α SVM
1	IPM fault (smaller units) or input overcurrent (larger units)	Same as αi	Internal cooling fan stopped	Cooling fan stopped
2	Control circuit cooling fan stopped	Same as αi	Control power undervoltage	Control power undervoltage
5	DC link capacitor did not charge in time	Same as αi	DC link undervoltage	DC link undervoltage
6	Control power supply voltage low	Input power open phase	Inverter overheat	Not used
7	DC link voltage too high	Same as αi	Not used	Not used
8	Excessive regenerative power (PSMR only)	Not used	L axis power stage (CNC number decides which alarm)	Abnormal current

The same Fanuc servo alarm codes read on four different modules.

Row 6 is the one that bites. On an α PSM, 06 means a phase is missing on the input. An αi PSM shows a missing phase as E, and its 6 means the control supply voltage sagged. Learn on α, walk up to an αi PSM showing 6, and you spend an hour hunting for a blown input fuse that isn't blown.

The dot is the other trap in Fanuc servo alarm codes, and it changes meaning three times. On an αi PSM a dot after the digit is a warning: the module keeps running and the alarm is on its way. An α SVM uses the same dot to turn an abnormal current alarm (8, 9, A, b, C, d, E) into an IPM alarm. And on an αi SVM, 8., 9. and A. belong to the power stage of axis L, M or N, and the display can't tell you which alarm it is.

The CNC number tells you which 8 you have

On 16i, 18i, 20i, 21i, 0i and Power Mate i controls, most amplifier alarms also come up on the screen with a number. Fanuc servo alarm codes on the display and that number on the screen are two halves of one message. An 8 on an αi SVM, with or without the dot, is trouble in the L axis power stage, and the screen says which kind:

·       449 is an IPM alarm.

·       600 is a DC link current alarm.

·       603 is IPM overheat.

The first two send you to the motor leads test further down. 603 is a cooling problem, and pulling leads won't find it. Same story for 9 on the M axis and A on the N axis. A 15i numbers its alarms differently, so don't map these onto a 15i screen.

αi SVM codes and what I check first

LED	CNC alarm	Meaning	Check first
1	444	Internal cooling fan stopped	Debris in the fan, fan connector, faceplate seated; then the fan, then the SVM
2	434	Control power undervoltage	Three-phase input at least 85% of rated; 24 V from the PSM normally 22.8 V or more; CXA2A/CXA2B cable
5	435	DC link undervoltage	DC link bar screws tight; several modules alarming means the PSM side; one module only: reseat its faceplate, then suspect that SVM
6	602	Inverter overheat	Motor load against its continuous rating; cabinet fans and filters; ambient temperature; faceplate seated
F	601	Radiator (heat sink) fan stopped	Debris, fan connector, faceplate seated; then the fan, then the SVM
P	604	Communication error between amplifier and module	CXA2A/CXA2B connector and cable; then the control board, then the SVM
8., 9., A.	449	IPM alarm, axis L, M or N	Faceplate seated, then the leads-off test below
8., 9., A.	603	IPM alarm (overheat), axis L, M or N	Heat sink fan running; motor load; cabinet cooling; ambient temperature
8 (or 8.), 9., A.	600	DC link current alarm, axis L, M or N	Leads-off test below
b, c, d	438	Motor current alarm, axis L, M or N	Servo parameters and motor ID first, then the leads-off test
Blinking "-"	Not listed	Abnormal control power supply	JF cable split test (below)
U	Not listed	FSSB communication error, COP10B side	See U and L below
L	Not listed	FSSB communication error, COP10A side	See U and L below

αi SVM Fanuc servo alarm codes with the matching CNC alarm on 16i, 18i, 20i, 21i, 0i and Power Mate i.

Half the rows share one first step: push the faceplate in. The control board rides on the front of the module, and when it isn't fully home you can get fan, DC link, overheat and power stage alarms out of a module whose fans and power stage are fine. Ten seconds. Do it first.

On a 5, count the modules. DC link undervoltage on one SVM only points at that SVM: its DC link bar screws, then the faceplate, then the module. The same alarm on several modules at once goes back to the PSM and the incoming supply.

A 2 needs a meter, not a guess. If the three-phase input is at least 85% of rated and the 24 V coming down the CXA2 cable reads 22.8 V or more, look hard at that cable and its seating before blaming the SVM.

A blinking dash is not the steady "waiting" dash. It means the control supply is abnormal, and the split is clean. Unplug the JF cable at the SVM and power up. Still blinking, it's the SVM. Stops, plug it back in at the SVM and unplug the other end at the motor instead. Still blinking now, the cable. Stops, the Pulsecoder in the motor.

For b, c and d, start with the parameters, not the hardware. Check that 2004, 2040 and 2041 hold the standard values for that motor on a 16i, 18i, 20i, 21i or 0i (a 15i uses 1809, 1852 and 1853), and that the motor ID is set right. If it only trips on hard acceleration, stretch the acceleration time constant and see whether it goes away.

8, 9, A, b, c, d: pull the motor leads before you pull the amp

These are the Fanuc servo alarm codes where an SVM gets replaced for what turns out to be a motor or cable problem. The test that tells them apart takes a few minutes:

1.    Push the faceplate fully in and power up again.

2.    Disconnect the motor power leads at the SVM and release E-stop. If the alarm comes back with no motor connected, it's the SVM.

3.    No alarm? With the leads still off, meg PE to U, V and W on those leads. Insulation normal points back at the SVM.

4.    Insulation low? Separate the cable from the motor and test each one. Replace whichever fails.

Before your hands go near those terminals, the DC link charge LED on the SVM should be off. What test voltage to use and which reading actually counts as a failure, I went through in how to meg a motor.

An α SVM with a dotted code gets one extra step at the top: wait about ten minutes, then release E-stop again. If the IPM alarm doesn't come back, it was overheating. Look at ambient temperature and motor load before anything else.

There's one pattern none of these steps catch, and it comes up more than you'd think: an amp that alarms on a cold morning and runs all day once it's warm. Every check passes, because the fault isn't there while you're checking. I stopped chasing those at the panel a long time ago. That module goes out for repair or gets swapped.

The leads-off test for 8, 9, A and b, c, d. It decides between the SVM, the motor and the cable before anything gets ordered.

U and L: look where L turns into U

Of all the Fanuc servo alarm codes on an SVM, U and L are the two about the FSSB fiber. The fiber comes out of the CNC into COP10B on the first SVM, leaves through COP10A to the next module's COP10B, and so on down the row. U is trouble on the COP10B side, L on the COP10A side.

Picture a row that reads L, L, U, U counting out from the CNC. The cable between the last L and the first U is the one I change first. If the code stays, the modules on either side of that cable are next, and for a U the servo card in the CNC is the last item on the list.

Not a fault: U flashes for a moment at every power up, then the display settles on a steady dash.

Reading U and L along the FSSB fiber. The break usually sits where the L modules end and the U modules begin.

αi PSM codes

The PSM's Fanuc servo alarm codes run 1 through 8, plus A, E and H, and every one of them has its own CNC alarm number.

LED	CNC alarm	Meaning	Check first
1	437	IPM fault (smaller units) or input circuit overcurrent (larger units)	Input phase balance; AC reactor matches this PSM; then IPM or IGBT
2	443	Control circuit cooling fan stopped	The fan
3	431	Main circuit heat sink too hot (overload)	Main circuit fan; dust; load; faceplate seated
4	433	DC link voltage dropped	Power dips; low input voltage; main power dropped while E-stop was released (sequence)
5	442	DC link capacitor not charged in time (precharge failure)	Too many modules for this PSM; DC link short; precharge resistor
6	432	Control power supply voltage low	Input voltage
7	439	DC link overvoltage	More regeneration than the PSM can return; supply impedance too high; main power dropped while E-stop was released
8 (PSMR)	605	Excessive regenerative power	Regenerative resistor rating; regenerative circuit
A	606	Radiator (heat sink) fan stopped	The fan; faceplate seated
E	607	Open phase on the input	Supply voltage, then connections
H (PSMR)	440	Regenerative resistor overheating (excessive deceleration power)	Resistor wiring and rating; fewer acceleration cycles; resistor fan

αi PSM Fanuc servo alarm codes with the matching CNC alarm. PSMR is the version with a regenerative resistor.

Remember the dot here. A 2 with a dot after it is a warning, so the machine keeps running while the fan dies. Cheapest moment you'll ever get to change that fan.

When the PSM sits on a steady dash and the main contactor never pulls in, this is my order:

·       E-stop actually released, and the ESP signal on CX4 of the PSM wired right.

·       CXA2A on the PSM going to CXA2B on the next module, every CXA2 cable seated and in the right socket.

·       Coil voltage on the contactor itself.

·       The PSM's MCC relay contact on CX3, pins 1 and 3. It should close and open.

·       Only then a module.

VRDY OFF on the screen, no code on the amplifiers

Sometimes the CNC throws VRDY OFF and there are no Fanuc servo alarm codes on any display. Four things to look at: the CXA2A/CXA2B cable between modules, the E-stop signal on CX4 at the PSM, whether an axis detach setting is blocking the MCON ready command from the CNC, and the SVM control board (faceplate in first, then the board itself).

There's a faster route on 16i, 18i, 21i, 0i and Power Mate i, if the servo software is new enough to support it. Diagnostic 358 shows how far the amplifier got through its start-up. Convert the value to binary. Bits 5 through 14 turn to 1 one after another as the amplifier comes up, and after a clean start all of them are 1. The first one still at 0 is where it stopped.

Bit	Signal	If this is the first 0, look at
6	*ESP	The emergency stop circuit
7, 8, 9	MCON	The ready command from the CNC through the amplifier to the PSM
10	CRDY	The PSM (converter) not ready
11	RLY	Dynamic brake relay not energizing
12	INTL	Dynamic brake relay not de-energizing (interlock)
13	DRDY	The servo amplifier not ready

Diagnostic 358 on 16i, 18i, 21i, 0i and Power Mate i.

Spindle amp codes that get blamed on the servo side

The SPM shows two-digit codes from its own list, separate from the Fanuc servo alarm codes above. Three of them turn up in servo troubleshooting far more than they should:

·       24 (CNC alarm 749) is a serial data error between the CNC and the spindle amp. It also comes on when the CNC is powered off, so a 24 on a cabinet with the control down means nothing. With the CNC running, look for a signal cable run too long or bundled with motor power leads, then the cable itself (or the optical adapter, if there is one), then the SPM, then the CNC.

·       31 (9031 or 7n31) is a motor lock alarm: the motor didn't turn as commanded. If the spindle crawls, turn it by hand and watch motor speed on the diagnosis screen. No reading means the feedback cable or the sensor. If it doesn't turn at all, check the spindle lock sequence and the power leads.

·       01 (9001 or 7n01) is motor overheat. Spindle motor cooling fan first.

The SPM also repeats PSM faults under its own numbers:

PSM shows	SPM shows	On the CNC	What it is
1	30	9030 / 7n30	Converter input circuit overcurrent
2	59	9059 / 7n59	Converter cooling fan stopped
3	58	9058 / 7n58	Converter main circuit overload
4	51	9051 / 7n51	Converter DC link undervoltage
5	33	9033 / 7n33	DC link precharge failure
7	11	9011 / 7n11	DC link overvoltage
E	04	9004 / 7n04	Open phase on the converter main supply
H	57	9057 / 7n57	Excessive deceleration power

PSM faults the αi spindle amp reports again under its own code, on 16i, 18i, 20i, 21i, 0i and Power Mate i. The n is the spindle number.

Say the PSM shows 2 and the spindle amp shows 59. That's one stopped fan in the PSM, reported twice. Leave the spindle amp alone.

Swapping a module: match the whole number

Two things catch people on a swap.

The full order number, not just the model. An α SVM3-12/12/12 is A06B-6079-H301 on most controls and A06B-6080-H301 on a Series 20 or 21-GA. Both use the same A16B-2202-0500 wiring board, but the control board is A20B-2001-0750 on one and A20B-2001-0760 on the other. Close isn't close enough.

Then the absolute encoders. Their backup battery reaches the Pulsecoders through the SVM side, from a battery case on the SVM or down the CXA2 chain. An αi Pulsecoder has a backup capacitor that holds position for roughly ten minutes with the battery disconnected. A standard α Pulsecoder has none. So on α, and on αi when the swap runs long, plan on setting the reference position again.

FAQ

What does alarm 7 mean on a Fanuc servo amplifier?

On a power supply module it's DC link voltage too high (07 on an α PSM), usually more regeneration than the PSM can handle or a supply with too much impedance. Neither α nor αi SVMs use 7, so check which module is actually showing it.

What does alarm 2 mean on a Fanuc servo amp?

Depends on the module. On a PSM it's the control circuit cooling fan. On an SVM it's control power undervoltage: check the incoming three-phase, the 24 V from the PSM (22.8 V or more) and the CXA2 cable.

What is alarm 24 on a Fanuc spindle amplifier?

A serial data error between the CNC and the spindle amp. It also shows when the CNC is off. With the control running, it's noise, the cable, the SPM or the CNC, in that order.

Are Fanuc servo alarm codes the same on alpha and alpha i amplifiers?

Mostly, with exceptions that matter. The α PSM shows two digits and its 06 is an open input phase, while an αi PSM uses 6 for low control voltage and E for the open phase. αi SVMs also have codes the α never had, like 6, F, P, U and L.

Do these Fanuc servo alarm codes apply to beta amplifiers?

No. β units have their own code list, so read their display against their own documentation, not these tables.

Before you order a replacement module

Most Fanuc servo alarm codes in these tables end at a fan, a cable, a faceplate that isn't seated or the incoming supply, and none of those needs a new amplifier. When the leads-off test or the JF split does point at the module, IQElectro stocks both generations: αi servo amps such as the A06B-6114-H208 (SVM2-40/80i), αi power supply modules like the A06B-6110-H011 (PSM-11i), spindle amps like the A06B-6111-H011#H550 (SPM-11i), and α units including the A06B-6079-H206 (SVM2-40/40) and the A06B-6077-H106 (PSM-5.5). Match the full order number to the one that came out of the cabinet.