Is it a copyright-protected (DRM) file? Copyright-protected files cannot be played.
Also, the file may be corrupted, the file name was changed, or the file was moved. Check the status icon in the library.
For details, refer to “Managing music files” in “rekordbox software manual”. Obtain “rekordbox software manual” from the rekordbox web site.
http://rekordbox.com

Check the following:

1. Is the [MASTER LEVEL] control set to the proper position? Adjust the [MASTER LEVEL] control according to the connecting device.
2. Is the [HEADPHONES LEVEL] control set to the proper
position? Adjust the [HEADPHONES LEVEL] control according to the connecting device.
3. Is the [TRIM] control set to the proper position? Adjust the [TRIM] control so that the channel level indicator’s orange indicator lights at the peak level.

It is possible that the language of the song information is not supported by the connected DJ software. For information on languages supported by the DJ software, see your DJ software's operating instructions.

Since MP3 and ACC format files support ID3 tags used to embed song information, the artist names and album names can be read and displayed from the files. On the other hand, WAV and AIFF files do not support the ID3 tag, thus artist names and album names cannot be read and displayed from the files. To display song information, you must use the Edit item to edit the song information. With the Import from iTunes function, since the file information held by the iTunes library is referenced, song information is displayed even when included as part of WAV or AIFF files.

When the master tempo mode is turned on, key control processing is performed to keep the key constant, so the sound quality changes. This change in sound quality is normal considering the product’s specifications; it is not a malfunction.

Is the crossfader curve setting appropriate?
The crossfader curve setting can be changed in [Preferences] of rekordbox. For details, see rekordbox Operating Instructions.
rekordbox Operating Instructions can be checked on the support site on rekordbox.com.

Also, the crossfader cut lag can be adjusted to any of 53 levels in [Preferences] of rekordbox. For details, see this unit’s Operating Instructions. The Operating Instructions can be checked on the support page.

Check the following.
(1) Is the jog dial dirty?
　An electrostatic system is used as the detection method in the jog dial of this unit.
　Therefore, if the jog dial is dirty or the user has hand cream on their hands, it may not respond.
　When cleaning the jog dial, wipe it with a dry soft cloth.
　If it is very dirty, prepare a 5:1 or 6:1 solution of water and neutral detergent, immerse a soft cloth in the solution and wring it out well, wipe off the dirt with the cloth, and then wipe the jog dial with a dry cloth.

(2) Does the jog dial respond when touched while touching some other metal part of the unit?
-If the jog dial does not respond to touching, the unit may be malfunctioning
　If the jog dial responds to touching, the unit may be affected by a connected device.

-The jog dial’s touch sensor may not work properly depending on the computer (e.g., a BTO computer) or amplifier connected to the unit or the connection environment (ground connection status, etc.) of that device.

Is crossfader assignment set properly?
Crossfader assignment can be set for each deck in rekordbox.
For details, refer to “Mixer” in rekordbox Operating Instructions. rekordbox Operating Instructions can be checked on rekordbox.com.
http://rekordbox.com

When the unit is connected to a computer (Windows) for the first time or when it is connected to a different USB port on the computer, the message "Installing device driver software" may appear on the computer screen. Wait until the message "Your devices are ready to use." message appears.

-In the middle of installation, the message "Connect to Windows Update to search for hardware? ] message may appear. In that case, select [No, not this time] and then click [Next] to continue installation.

When the [Back spin length] setting in [Preferences] of rekordbox is set to [Normal] or [Long], the behavior may be as if the jog dial continues to be rotated even after it has stopped being rotated due to software processing. To match the playback position to the actual rotation state of the jog dial, set the [Back spin length] setting to [Short].
The procedure to set the [Back spin length] setting to [Short] is as follows.
1) Connect this unit to your computer via a USB cable.
2) Change the [Back spin length] setting in [Preferences] > [Controllers] > [Decks] of rekordbox to [Short].

Deck 1 and deck 2 of rekordbox dj are supported by this unit. Deck 3 and deck 4 can be operated on the computer or by connecting a separate general MIDI controller.
To operate rekordbox dj with other than this unit, use the MIDI assignment function of rekordbox dj.
For information regarding the MIDI assignment function of rekordbox dj, refer to “MIDI LEARN Function Operation Guide.”
http://rekordbox.com

1. Is the included USB properly connected?
Connect the included USB cable properly.
2. Is the supply enough from the USB terminal on your computer connected to this unit?
Connect to the USB terminal which can supplies enough.
If other devices which is not used for DJ play is connected to your computer, disconnect them.
Do not use your computer running on its battery. Supply power to the computer from an AC power supply.

This unit may not operate properly due to static electricity or other external influences. In this case, please disconnect the USB cable and connect it again.

If this unit operates abnormally or does not start, disconnect the USB cable, wait a while, and then connect it again.
If the symptoms still occur, put in a request for repairs.

There are various possible causes. Check the following.

(1) Are the connection cables connected properly? Connect the connection cables properly. For details, see “Connections” in the Operating Instructions.
(2) Are terminals or plugs dirty? Wipe any dirt off terminals and plugs before making connections.
(3) Are the connected powered speakers, etc., set properly?
Set the external input selector and volume of the powered speakers, etc. properly.
(4) Is the driver software set properly? Set the driver properly. For details, refer to “Preparing rekordbox” in the Operating Instructions.

(5) Check the procedure to output audio.
[Outputting audio]

The following describes the procedure for outputting the channel 1 sound as an example.

Output audio from the [MASTER] output terminals.
Set the volume of the device connected to the [MASTER] output terminals (power amplifier, powered speakers, etc.) to an appropriate level. Note that the output sound may be loud if the volume is set too high.

1. Set the positions of the controls, etc., as shown below.
[MASTER LEVEL]: Turned fully counterclockwise
[TRIM]: Turned fully counterclockwise
[Channel fader]: Moved towards you

2. Play the track.
3. Move the channel fader ([Image No. 76] Figure 7) backward.
4. Turn the [TRIM] control ([Image No. 76] Figure 2).
Adjust [TRIM] so that the orange indicator on the channel level indicator lights at the peak level.
5. Turn the [MASTER LEVEL] control ([Image No. 76] Figure 9) to adjust the speaker audio level.

Output audio from the headphones.
1. Set the positions of the controls as shown below.
[HEADPHONE LEVEL]: Turned fully counterclockwise

2. Press the headphones [CUE] button of channel 1.
3. Turn the [HEADPHONE LEVEL] control ([Image No. 76] Figure 17).
Adjust the sound level output from the headphones to an appropriate level.

1. Are you using applications other than rekordbox at the same time?
Quit other applications to decrease the load on the computer.
If operation continues to be unstable, try disabling the wireless network or other wireless connections, antivirus software, the screen saver, the low power mode, etc.
2. When other USB devices are connected to the computer.
Disconnect other USB devices from the computer.
When another USB audio device is connected to the computer at the same time, it may not operate or be recognized normally.
3. Are you using a USB hub? USB hubs cannot be used. Connect the computer and this unit directly using the included USB cable.
4. There is a problem with the USB A port to which this unit is connected.
If your computer has multiple USB A ports, try connecting this unit to a different USB port.
5. Is this unit connected to a notebook computer running on its battery?
Supply power to the notebook computer from an AC power supply.
(Depending on the notebook computer’s settings, the low power mode may be set when the computer is running on its battery, automatically reducing the performance of the computer itself.)

Is a cue set?
Set the cue.

1. Is rekordbox set to the [EXPORT] mode?
Click the area indicated by [EXPORT] at the upper left of the screen, and then select [PERFORMANCE].
2. Has rekordbox dj been activated?
Enter the rekordbox dj license key included with this unit on the [Preferences] > [License] screen of rekordbox. For details on the activation, see the rekordbox introduction.
rekordbox introduction can be obtained from the rekordbox website.
http://rekordbox.com

If audio is not output or you cannot control rekordbox when using Windows, please see here to solve the problems.

The DDJ-400 operates with power supplied from USB. When the power supply decreases, this unit places a limit on the volume of the headphone output. In this case, please remove other USB devices connected to the USB terminal of your computer. Please do not use your laptop running on the battery. Supply power from an AC power supply.

Setting the master output’s peak limiter level is possible only when using rekordbox dj. It does not work for other DJ applications such as Traktor.

Switching monaural sound or stereo sound is possible only when using rekordbox dj. It does not work for other DJ applications such as Traktor.