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The FANUC SV0442 (PS “Pre-charge failure”) alarm indicates that the DC link capacitor in the power supply/module did not pre‑charge to the required voltage within the specified time, so the converter pre‑charge sequence failed .

What it means

– The alarm is issued when the DC link of the power module is not normally pre‑charged during the converter’s start sequence, typically flagged as AL‑05 on the Alpha iPS/PSM unit and mirrored to CNC as SV0442/SP9033 .  

 “Converter: DC link precharge failure” and note it can also appear after momentary power interruptions as the DC bus fails to reach threshold in time 

 Primary causes 

– No or incorrect three‑phase input to the power module main circuit and monitor input (L1/L2/L3 and CX48), including an open phase, wrong phase sequence, loose wiring, or tripped breaker/contactor .  

– Input voltage out of spec, including low line, imbalance, or overvoltage conditions that prevent proper timing of the pre‑charge circuit.  

– Magnetic contactor is not turning on, or ON timing sequence from the PMC is inappropriate, causing the pre‑charge window to expire before the contactor applies power .  

– Too many connected servo/spindle amplifiers on one power supply leading to extended charge time beyond the allowed limit .  

– Mismatch between the power supply monitor connector (CX48) voltage/phase and the actual main input terminals (L1/L2/L3) .  

– DC link short or internal fault in the DC link/pre‑charge path of the power supply module (e.g., failed pre‑charge resistor/relay or power section) .

Recommended checks

1- Verify three‑phase input presence, voltage level, balance, and phase sequence at both main input and the CX48 monitor; correct any opens, reversals, or low voltage issues .  

2- Inspect and test the upstream breaker, magnetic contactor, and all cabling to the power module; ensure the Magnetic contactor (MCC) turns ON promptly after E‑stop release per the prescribed sequence .  

3- Review PMC ladder for contactor control timing; reduce delays so pre‑charge completes within the allowable time window .  

4- Reduce the number of simultaneously connected amplifiers or distribute load if the bus cannot pre‑charge in time with the current configuration .  

5- If wiring, voltage, sequence, and load are correct, suspect a DC link/pre‑charge hardware fault and evaluate/repair or replace the power module; .

6-For checking purpose you can short the connector CX3 after removing it from Drive and short it via external wire.

You have to pull out connector CX3 From combo drive and short the pull out connector pin 1 & 3 with the help of External wire 0.5 mm .

If combo drive’s internal relay is faulty in that case the supply which goes through the CX3 relay can not come back to the wire KMCC so pullout the  connector CX3 in emergency state and short the pullout connector pin 1 and 3 with the help of External 0.5 mm  wire .If drive’s internal relay CX3 is faulty then after shorting the CX3 connect pin 1 & 3 MCC contactor will be hold and 200 v ac supply goes to Combi drive through AC line Reactor module after releasing the emergency push button.

Fanuc CX3 connector is the internal relay of Fanuc drive.Fanuc CX3 connector is Normally Open circuit when fanuc drive is in healthy state then CX3 become Normally Close circuit and the supply goes through the wire P (CX3) in below image come through the wire MCC in below image.

Machine Tool builder apply 24 V dc supply through wire P . You have to check this supply in your machine’s electrical drawing or you can also check it by plug out the connector CX3 and check with the help of Multimeter.

For example in below image 24 V DC is applied on CX3 Connector. In below image 24 V DC supply is applied on wire P (below image) then when machine is in healthy condition then 24 V DC supply come out on wire MCC .You can check this supply on MCC terminal of your machine.

Then this 24 V DC energize coil A1-A2 of MCC relay & (200-230 V AC) (depending on various country ) energized the MCC Contactor coil which is shown in following image.

When MCC contactor energized then 3 Phase incoming supply(R3A,Y3A,B3A)(200 V AC) goes through MCC contactor & (R3B,Y3B,B3B) goes to Fanuc combo drive through Fanuc AC line reactor module and drive start to pre charge its DC Link. If there is any error in the above circuit then alarm Pre Charge failure occurs.

If machine is not in healthy condition then this supply will not come out.

To understand how Fanuc CX3 connector works we should understand the below image. This is another Machine Tool Builder electrical drawing in which MTB Apply 24 V dc to CX3 connector.

For example Machine Tool Builder apply 24 v dc through wire L then in healthy condition this 24 v dc goes through internal relay CX3 and come out through wire KMCC (24 V dc).This 24 v dc supply energize coil (A1-A2) of MCC relay & (200-230 )V AC (depending on different countries voltage supply) goes through this relay to MCC contactor Coil (A1-A2)and energize the MCC contactor .

Incoming supply 200 V AC on MCC contactor will go to Fanuc combi drive through Fanuc AC Line reactor Module.

You may also check 200 V AC supply at the incoming of Fanuc AC Line reactor Module. Many times due to problem in MCC contactor 200 V Ac does not go to Fanuc AC Line reactor Module and alarm Generated.

Many times due to problem in Fanuc AC Line Reactor module also 200 V AC does not go to Fanuc drive and Pre charge failure alarm occurs.

In some cases( 200 -230 )V ac (depending on different countries voltage supply) supply is going to MCC contactor coil but contactor is not getting hold then there may be problem in MCC contactor .In such case you need to change the MCC contactor.

If drive is not combo then you have to check that Seven segment display of Power supply module is On or Off .If Seven segment display is off then you have to check fuse inside the Power supply module.

Fuse inside the Power supply module is ok or Not.

if Fuse is blown away then Seven Segment display will not be on and dc link alarm will be appeared.

7:If Shorting the CX3 connector with the help of external  wire and mcc contactor is getting hold and 200 V AC supply is going to PSM (Combi drive) and still alarm is occurring then there may be problem in PSM or Combi drive then Contact to nearest Fanuc Center.

Disclaimer: The blogs shared on CNC machines are created purely for *educational purposes*. Their intent is to help readers understand CNC controls, alarms, diagnostics, and general troubleshooting methods. We strictly avoid any copyright violations, and all explanations are written only for learning and knowledge-sharing.  These blogs should not be considered as official repair or service manuals. For detailed instructions, critical repairs, or advanced troubleshooting, it is always necessary to contact and work under the guidance of the respective *machine manufacturer* or *CNC controller support team*.  The content provided is focused only on *diagnosis and awareness*. We do not take responsibility for any kind of damage, error, or malfunction that may occur if someone directly applies the information shared here without proper technical supervision.#