2001 Kia Shuma ENGINE YELLOW LIGHT ON

At first, sorry for my poor english. When engine yellow light is on, engine looses power. Now a days yellow ligth remains on frequently. Some body checked it and indicates fuel mixt is poor. All sensors have been changed, including spark plugs cables and engine coils. The engine never fails but runs under power. Compressions ok, injectors clean and ok, vaccum tube ok, fuel pump ok, fuel filter ok, spark plugs ok, air filter ok, battery ok. Fuel consumption is low. Some times yellow light switces of, and the engine runs smoother and power increases.
Thank you very much,
Antonio Lopes