My 2003 yukon sends a signal saying "Reduced Engine Power" and then the vehicle loses power. What casuses this, how can it be fixed?

when mine does this I have found that if you unplug the wiring harness on the side of the throttle body and count to 10 and plug it back in it temporarily solves the problem for about 3 months. If this helps you the engine light will stay lit for approximately 3 starting cycles the returns to normal operation. I suspect a bad computer control on the left side of the firewall where the other end of the wiring harness connects.

I have same issue with my 2003 Yukon xl w/ 5.3. Check the owners manual, and said and issue w/ the coolant. So who is right??????

Low Power is a safety in case of brakes, throttle or overheat. Can happen at any time like for me in process of a left turn into oncoming traffic. Codes indicated foot throttle pedal 'drive by wire' sending unit. I cleaned throttle body connector .. fixed it for 2 days, then again for a week, then bought a scanner and read code for throttle pedal sender (a type of variable potentiometer likely with foot position connected to computer to throttle body). Changed foot pedal position sensor (you'll need a 1/4 " drive 10mm wobble socket ti get behind a restriction). After 3 weeks it occurs again. Now I need to think about other items like the computer itself or a new throttle body and new connector.

Replace your throttle body sensor fixed mine

Throttle body sensor and / or accelerator pedal position sensor (601-199) I changed the TPS first and continued to have the same problem. I then replaced the APPS and it fixed the problem... Good wishes guys