Tire pressure drops to 28 overnight, but never lower.

If the leak is small, it would drop fast when pressure is high and the leak would slow down after that. If you allow it to sit longer, you would notice the pressure dropping further or the tire pressure sensor is not working correctly here are the instructions on how to change them out in the diagrams below. Check out the diagrams (Below). Please let us know what happens.

Here is the info you requested

TIRE PRESSURE MONITOR DESCRIPTION AND OPERATION

The tire pressure monitor (TPM) system warns the driver when a significant loss, or gain of tire pressure occurs in any of the 4 tires and allows the driver to display the individual tire pressures and their locations on the driver information center (DIC).

The system uses the engine control module (ECM), body control module (BCM), instrument panel cluster (IPC), DIC, remote control door lock receiver (RCDLR), a radio frequency (RF) transmitting pressure sensor in each wheel/tire assembly, and the serial data circuit to perform the system functions.

When the vehicle is stationary, the sensors internal accelerometer is inactive, which puts the sensors into a stationary state. In this state the sensors sample tire pressure once every 30 seconds and do not transmit at all if the tire pressure does not change.
As vehicle speed increases, centrifugal force activates the sensors internal accelerometer causing the sensors to go into rolling mode. In this mode the sensors sample tire pressure once every 30 second and transmit in rolling mode once every 60 seconds. The RCDLR receives and translates the data contained in each sensors RF
transmission into sensor presence, sensor mode, and tire pressure. The RCDLR sends the tire pressure and tire location data to the DIC via the serial data circuit where they are displayed as follows:

XX TIRE OK XX PSI

XX TIRE LOW XX PSI

XX TIRE HIGH XX PSI

The sensors continuously compare their last pressure sample to their current pressure sample and will transmit in re-measure mode if a 1.2 psi change in tire pressure has been detected in either a stationary, or rolling state. When the TPM system detects a
significant loss, or gain of tire pressure, the CHECK TIRE PRESSURE message is displayed on the DIC and the low tire pressure indicator is displayed on the IPC. Both the message and indicator can be cleared by adjusting the tire pressures to the recommended kPa/psi. The sensors pressure range is 0-116 psi. The sensors pressure
accuracy from -10 to +70° C is +/- 13.7 kPa (2 psi).

The RCDLR has the ability to detect malfunctions within the
TPM system. Any malfunction detected will cause the DIC to display the SERVICE TIRE MONITOR message.