This depends on the model of the sensor and how it is cabled. If you have one of the sensors that built in failopen capabilties then the connectivity will behave as if the sensor is not there at all.
If it's one that requires an external fail open kit and you cable it directly to the sensor, then there will be no connectivity on the cable.
Specifically, the models that have built in fail open ports are I2700, M1250, M1450, M2850, M2950 and NS9x00.
The sensor ports cannot be enabled until the sensor is initialized, so the best bet is to get the sensor up and running and get a sigset in place before cabling the device.
If you have one of the sensors that does not have internal fail open, an external fail open kit can be used. If the sensor port is down, these kits will default to passing traffic between the network ports.
the model are m1450 and they are pre-cable in-line on the failed open port. So you are saying that it will not cut traffic if we power them up and put in the inital config ?
correct. The ports will be disabled on a M1450 during initial config