[Mark McLeod]

That's what you usually see when you use a separate battery to power the IR board. With two different batteries the grounds are not common and the signal pins stay high.

You could get around it with a check of all the pins, such as:

if (rc_dig_in13 == 0 || rc_dig_in14 == 0 || rc_dig_in15 == 0 || rc_dig_in16 == 0)
{
    if (rc_dig_in13 == 1)
    {
        //Drive away
    }
}