If you are referring to the Check Engine light it is on because the computer has set a fault code due to a problem with the emissions system.
The 2005 Ford Focus has an inline 4 engine.
The 2005 Ford Focus has a 2.0 L base engine size.
The 2005 Ford Focus's engine produces 136 hp @ 6000 rpm.
The 2005 Ford Focus's engine produces 133 ft-lbs. @ 4500 rpm.
It means... take it to Auto Zone and they will test it for FREE.
Check engine?
Yep.
The 2005 Ford focus has a timing chain not a belt. No maintenance is required.
Unhook the battery, overnight, and in the morning, rehook it. Make sure poles are clean.
My experience has been that if you disconnect the battery cables and leave them off for a minute, then reconnect them, the service engine light should have reset.
The 2.0 liter engine in a 2005 Ford Focus has a timing CHAIN Helpfull
On the bottom of the engine, Einstein!