To put the headphones in pairing mode start with the headset off, then press and hold the "S" button down for about 7 seconds until it blinks blue and red as this indicates the headset is pairing mode and THEN do a device discovery search on the device you want to connect the headphones to. Here's a how to video
NOTE: The model in the video may be different but for most of our models the concept is the same.
** FOR THE PXC 550 please see page 21 of the manual here for the pairing process **
If prompted for a password/PIN during the connection process the default number is "0000".
In a situation where the headset had previously been connected to the device and was working and now the headset will not connect to the device try deleting/removing the Bluetooth connection from the device, then turn the headphones and device completely off, then turn the device on and then pair the headphones in the manner mentioned above.
As long as the device that the Sennheiser headset is being connected to supports the appropriate Bluetooth version (usually 2.1 and EDR for Sennheiser products) or higher and the appropriate Bluetooth profiles (A2DP, HSP, HPF, AVRCP) there should be no connectivity issues.
Please ensure that the device is supporting the appropriate Bluetooth Version and Profile and make sure that the settings on the device that the headphones are being connected to are adjusted to recognize headphones/microphones (ie: on some devices the "Hands Free Profile" needs to be activated). NOTE: Sometimes an update will have occurred on the device that the headphones are being connected to and it requires the device to be rebooted (ie: turned completely off and then back on) before the headphones will connect.