When you only have one of the two wires connected to an IN-4, this can cause the LED to go red. The sensor will regularly check if it is able to deliver the appropriate current level for the given CO2 level. If that is not possible, LED will turn red. You can solve the problem by adding a resistor between the output (black wire, 0-5000ppm) and 0V (blue wire). Size of the resistor will depend on the supply voltage. For 24V a 500R resistor will probably do the trick. (This assumes you are using the 0-10,000 ppm (white) wire).