ford focus what malfunction is caused?

Which engine light?
The cars engine management computer constantly monitors all the sensors and actuators on the car.
If it receives a signal from any of them which is out of normal range, or which does not correlate with the signals from other sensors, or it receives no signal at all, it will log a fault code in its memory and switch on the Engine Management Light (EML) to warn you.
Given the sheer number of conditions which can put the light on it is usually essential to plug the car in to a diagnostics computer to see what fault codes have been stored.
Please note that although a fault code usually refers to a specific sensor it does NOT necessarily mean the sensor is faulty! For a decent technician the fault code is just the first step in the diagnostic process.

You dont say what year the car is but if it's out of warranty lots of independent garages have diagnostics equipment. They will usually be a lot cheaper than a Ford dealer.

In short, do what Ghost said, get it plugged in to diagnostics, I'm just typing here to look busy!!

bored at work.
June 2011

to know what problem's may have been caused it will have to be connected to a computer your local garage should have the equiptment to tell you what the problem(s) are

GHOST
June 2011