DIRTY MAF 

Clean your Mass Airflow Sensor with some MAF sensor cleaner or contact cleaner

vacum leaks around intake

Inspect your intake hoses and make sure non are leaking

 

Inspect also the vacuum caps on the manifold and make sure they are both on

 

dirty eccentric shaft sensor

Check if your eccentric shaft sensor is excessively dirty and clean it

OLd coils-plugs-wires

Check that they are perfectly clicked in place and in the correct order.

 

If your coils, wires or spark plugs are older than 4 or 5 years, It's a good Idea to replace them.

More power + better mpg

reset ecu

Sometimes is just an ECU error, and all you need to do is reset it by:

1) turn ignition on, but not the car

2) pump your brake 25 times fast and hard until the oil gauge bounces (it does not bounce on all models)

3) Ignition off, ignition on and start the car

low compression

If non of the above helps, do a compression test and if you have less than 5 BAR consider rebuilding your engine.