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The one on the PCU is the one that tells the ECU when the desired pressures are reached. There is also a pressure switch that allows the CTIS system to actually function once there is at least 90 psi in the wet air tank.
Pressure switch is mounted in the wet tank should be a circular connector with 2 or 3 sockets but only 2 are used. Try not to let the switch body twist when removing the connector it will destroy the switch.
Follow link to the service manual AXTS-0015 Page 3 shows about where the pressure switch is located
When the ignition is turned on and a fault occurs in just a few seconds, indicates the system had an electrical fault, PCU sensor or solenoid issue. When the system is normal until it check the pressures it indicates it did not like the result of the tire pressure check. If there are no air leaks and there is a flat tire you can push "run flat" to override the fault. If the system reaches any selected target pressure and the tires are still imbalanced it will log a 4 light fault. The system will read the tires as an average so for it to read imbalanced tires at a target same tires must be high and one or more are low. The best thing to do to check out any fix is to start with balanced tires.
To determine if the wheel valves are good.... manually inflate all the tires to >5psi above the Highway target. Disconnect the pressure switch. Start the truck and let the supply build up to 125psi. Plug in the pressure switch. If the system checks the tire pressure, goes to a solid light, and does not dump any air, the wheel valves are fine and the PCU is also working. Shut off the truck and dump the supply air down to 10psi below the tire pressures, start the truck, if the system check the pressures and starts dumping air the switch is probably stuck closed.
The PCU has the Pressure Transducer its function is to measure the atmospheric pressure at start up and will fault out the system if it is out of range or unplugged, second it measures the average tire pressures during a pressure check, third it measures the deflation pressure (relief valve) during a deflation and will log a fault if that pressure is out of range.
If the valve looks good and will hold high tire air...
If the system is in a hold until the pressure builds up to > 115, that is a good start. When the switch closes and the pressure check is complete, if the pressure had read correctly the light should go solid. Because it is still flashing while the tires are dumping it must be trying to inflate them so two things are going on.... It read the pressures lower than the tires and the control line between the PCU and the 3 QRVs has dropped below the tire pressure. If air is coming out the PCU cover vent during the leak down the problem is in the PCU, maybe a stuck cartridge. With a metal screwdriver make sure the deflate solenoid is not energized during the attempted inflation. You either have a large line leak between the PCU and the QRVs or a stuck PCU cartridge that you can try to clean, not too complicated no real tiny parts.
When the system checks the tire pressure it puts a short pulse of air into the air lines. The wheel valves will open at 6.0 psi and the tire pressure will begin to fill the lower air lines up to the QRVs. The upper air line from the PCU the top of the QRVs is filled by the PCU and acts differently. The lower lines will be fed air from the tires and will loos air slowly if there is a small seal leak, but the upper air line can drop pressure quickly if the system has not started the inflation. As soon as the upper line air pressure drops below the tire pressure, the QRVs will start dumping the tires. This is exactly how the system is designed to deflate, it opens the valves and drops the upper line pressure down to 12 psi and out comes the air. So if there is a decent size leak in the upper line it will dump air just like a deflation. If the pressure switch closes too early, lower than the tire pressure, the tires will dump.
