What does it mean when it says tire senser bad/.

Check the tire pressure with an accurate pressure gauge-start here possible a wheel pressure sensor/low battery

TIRE PRESSURE MONITORING SYSTEM
NOTE:The TPM system relearn procedure must be performed before servicing the system. See TIRE SENSOR RELEARN PROCEDURE under PROGRAMMING.

There are 4 conditions that will cause a Tire Pressure Monitoring System fault to be set. All fault codes are associated with a specific wheel location.
If the EVIC detects a non-transmitting sensor/transmitter in a road wheel for 10 minutes at a vehicle speed more than 25 MPH (40 KM/H).
When the EVIC detects a low pressure sensor/transmitter battery status for 7 consecutive ignition cycles.
When the EVIC detects any sensor transmitting at a shorter than expected interval.
When the EVIC detects no valid pressure sensor ID from the spare tire for 20 consecutive ignition cycles spaced at least one hour apart.
In each case, it will store an active fault code, request a chime, display SERVICE TIRE PRESS SYSTEM and display a dashed line at the wheel location on the graphic display, if the display is activated.
All tire pressure monitoring system faults are specific to one location. If a BATTERY LOW, SENSOR FAILURE fault is detected, the location will be displayed. The appropriate sensor/transmitter can then be replaced. If a single sensor/transmitter cannot be detected by the EVIC, replace that sensor/transmitter. If none of the sensor/transmitters can be detected, perform self-test. See TIRE PRESSURE SYSTEM TEST.
TIRE PRESSURE SYSTEM TEST
NOTE:Pressing the RESET button during the test will sound a beep and reset all the counter values back to zero.

The following test can be used to verify two functions. One, that the tire pressure sensors are transmitting properly and two, the EVIC module is receiving these transmissions accordingly.
Retrain the tire sensors. The tire sensors must be retrained in order to set the proper transmitting time cycle (once per minute), failure to retrain the sensors will cause a much slower transmitting time cycle (once an hour).
Using the STEP button on the overhead console, scroll to the blank display, then press the RESET button for five seconds, a beep will sound indicating the start of this test. The vehicle icon and transmission counters will now be displayed (same display as individual tire pressure except counters replace tire pressure values).
Upon entering the test mode, the EVIC will clear the sensor counter and each time a sensor signal for a road tire is received, the EVIC will update the counter value when vehicle is driven. Drive the vehicle at speeds more than 25 MPH (40 KM/H) for 10 minutes. The counter values should all read the same value. The test will continue until any EVIC button is pressed or until the ignition is cycled to OFF. The counters will increase by one each time a sensor signal is received by the EVIC (approximately 1 signal per minute from each wheel except the spare). If any of the road tires indicate a different value than another tire sensor, this is a sign of a problem. Replace the appropriate tire sensor and retest the system. This test will continue until any of the overhead console buttons are pressed or the ignition is turned off.