The red light is blinking and the system is not calling for heat.
If the red light is blinking it means that the controller is not reading the temperature sensor (wire terminals 13 and 14).
The slab sensor can be bypassed by connecting a 47 kOhms resistor between wire terminals 13 and 14 and a jumper wire between wire terminals 15 and 16. This way the controller will call for heat.
If only one electrical zone selected, and the controller is calling for heat , it will close the contacts between 3 and 4, 5 and 6, 7 and 8.
When the controller is calling for heat, there should be continuity between terminals 3 and 4, terminals 5 and 6 and terminals 7 and 8.
If the controller's display is blank, use a voltmeter to test for voltage on wires entering "L" and "N" terminals. If there is 120 Vac across the wires entering "L" and "N", then test the glass fuse with a volt/ohm meter. If the fuse is good, then the controller has failed. If the fuse is bad, try replacing it with a 800mA fuse of the same type. If the fuse fails again, the controller has failed or the controller is getting too high a voltage at the "L" & "N" (over 120Vac). Verify voltage of wires entering "L" & "N".