There are many things that can go wrong with your Ford Focus engine. Many common problems also trigger a "check engine" light to come on. The check engine light can alert you of various problems with the engine's sensors. However, some problems can be diagnosed without having to bother reading the trouble codes. While replacement parts can be purchased from most auto-parts stores, it's a good idea to figure out what's wrong with your engine first.
Start the engine and open the bonnet. If there is any white steam coming out around the mating surfaces of the engine, there is a head gasket leak. The head gasket will need to be replaced. If there is black smoke coming out of the tailpipe, especially when you accelerate, then your engine's air-to-fuel ratio is off. You are burning too much fuel. This may also trigger a check engine light warning relating to the oxygen sensors on your Focus.
Locate the on-board diagnostic (OBD) access port to the left of the fuse panel in the driver's side footwell.
Plug the OBD scan tool into the access port. This is a 16-pin plug and can only be oriented one way.
Turn on the scan tool and turn the ignition to the "On" position.
Press "Start" on the scan tool. This will retrieve the engine codes. When the display reads "OK," unplug the scan tool and shut off the ignition. Use your Focus' service manual to translate the trouble codes (or see Resources).