Not only that, but the data is processed weekly for the reports. If a station is not encoded for several days in a week, it will be noted and followed up on. It would not go on for multiple weeks without Nielsen and the station noting the change in the weeklies.My experience has been that if an encoder doesn't check in after a couple hours, Nielsen reaches out to the station to alert them that they may want to restart the encoder, or switch to their backup encoder. With that in mind, I find it hard to believe that an encoder was locked up for an entire book.
If they are subscribed to the advance data, the station would know even sooner. But Nielsen would sense it within days.
However, Nielsen only gets meter data when "ET Phones Home" when docked for recharging each night. They don't have instant moment to moment data.
The encoder itself is supposed to reach out and touch someone if it is failing, but that requires the box be able to talk; I don't know what senses a total failure.