Emissions warning indicator...? I didn't know the Focus had one, unless its something new that came in 2005. It could mean that your cat is on its way out or that your downstream O2 sensor is shot. Its also possible that you got a bad tank of gas when you filled up last.
What does the lamp look like? Are you getting the exclamation with the zigzag line around it? Or is it something else? Normally if either O2 sensor gives the computer a reading that's outside the acceptable range it just throws a CEL...its hard to imagine what else it would do, seeing as how the entire emissions control system is monitored by the O2 sensors.