A flashing supply light on any APS or EUR-5A control indicates a missing or broken High Temperature Limit sensor. This sensor is required to make the EUR-5A controller to work. The sensor is not polarized and must be connected to pins 1 and 2. Refer to figure 9 in the manual.
High Temperature Limit Sensor
The High Temperature limit sensor is connected across terminals 1 and 2. There is no polarity requirement. If the High Temperature limit sensor is not used, then a 470k ohm resistor must be connected in its place, across terminals 1 and 2 of the EUR–5A. This is necessary to allow the EUR–5A to operate without the sensor. If the circuit is left open, the Heat indicator on the EUR–5A panel will flash.