I washed the engine on my 2003 avalanche now the reduced power lite is on

I suggest you read the article at Reduced Engine Power Warning: What Does It Mean? - In The Garage with CarParts.com

Problem with the electronic throttle control system . Position sensors in the gas pedal assembly or throttle body , have it checked for code's .

https://www.youtube.com/watch?v=JECpaqW72ZI

have it scanned usually this is a problem with the throttle actuator

Code p2120 is accelerator pedal position sensor APP sensor 1 circuit apparently this is known as the reduced engine power syndrome and is quite common on the avalanche. There is no mechanical linkage from gas pedal to throttle body and it is quite common for the throttle position sensor located on throttle body to mess up and throw this code and also for one of the wires to the throttle position sensor to get hot become brittle and sometimes break. I would start there

gasses could build up and cause explosion. thaw slowly and then recharge.

you need to scan the computer that has a electronic throttle body and a electronic gas pedal one sensor goes bad and the system goes into limp mode

I'm an owner who had this problem on / off for a year. Several returns to dealership proved hopeless. Service shop devulged that '03 Avalanches had a overall electric problem that could not be detected. I legally forced dealership's hand...They replaced the entire wiring harness at no cost. No problem since. George

this is most likely a throttle body issue. accelerator pedal has position sensors on it but these trucks had some issues with the electric throttle body and harness plugging into it

DTC P0120 P0220 P1516 P2135 these should be the problem codes.
2003-2006 Chevrolet Avalanche, Corvette, Express, Monte Carlo, Silverado, SSR, Suburban, Tahoe, Trailblazer. for reduced engine power. p0120 is throttle position sensor, ( tps ). p0220 is relates to tps, p1516 tps is out of range. p2135 is a throttle body code, the throttle body needs replaced. but before you do it replace tps first to see if it takes care of the other codes. have a good day !!

Did your check engine light come on? To really determine the issue you need to hook a scanner up to your vehicle and pull the codes. Once you know the codes then you have the information for a diagnosis.