LabQuest shows that a sensor is connected, but nothing is really connected to that channel.

This information pertains to the original LabQuest.
If you have LabQuest 2, see LabQuest 2 FAQs and Troubleshooting

This is a hardware defect in the data acquisition board (DAQ). The unit needs to be repaired. Please contact Vernier Technical Support, via phone, 888.837.6437 or fill out an online request for support, http://www.vernier.com/forms/support/