Land Rover Repair: Engine Power Loss, coolant temperature sensor, mass air flow sensor

Question
I have a 2001 Discovery.  Recently the engine light has come on indicating cylinder misfires and system banks 1 & 2 too lean.  I have replaced the spark plugs.  When the gas pedal is pressed down hard the engine loses power or when going up a hill at high speeds I'll lose speed fast and cannot regain it until going down hill again.  I cannot accelerate fast anymore because the engine does not have the power for a fast acceleration.  What should I troubleshoot now?

Answer
So if the lean mixture is true, then 3 fault may occur that.
- Oxygen sensors faulty (not plausible for 2 sensors fails at one time)
- Mass air flow sensor measuring lower then the truth. Maybe very much air can go into the plenum chamber, from wrong location (i.e. cracked hose anywehere after the Airflow sensor)  Try to find any sucking noise...
- Coolant temperature sensor fault