My 2003 chevy avalanche has been having the REDUCED ENGINE POWER warning and the engine cant acelerate.

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

You have a problem with the electronic throttle control . You need to have it checked for codes , could be a few different thing's Pt 1 2007 Chevy 2500HD Reduced Engine Power Warning Fault Code P1516... P2138 APPS Accelerator Pedal Position Sensor Fix 3 5L Chevrolet Impala Lot's of codes for this , Drive by Wire Function and diagnostics by Wells

2003 Silverado 2500HD REDUCED ENGINE POWER - Discussion on Topix

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Reduced Engine Power Message Displayed on GM Vehicles

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Dec 10, 2012 - Tech Tip: Reduced Engine Power Message Displayed on GM Vehicles ... Model year 2008-'10 vehicles must have the engine control module (ECM) reprogrammed ... Do not drop the electronic throttle control components.

2003 Chevy Trailblazer-CHECK ENGINE LIGHT AND REDUCED ENGINE POWER ...

www.topix.com > California > San Diego County > Oceanside
Dec 23, 2011 - 2003 Chevy Trailblazer-CHECK ENGINE LIGHT AND REDUCEDENGINE POWER ... The reduced engine power ligth comes on and I lose power. ...Next, the fan clutch failed -- which is when he did an "above and beyond" .... yesterday I have not replaced anything bc they have told me nothing is wrong.

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

cam or crank sensor failing

Here's a link to your code and solutions Wilson.

what happened is it got water either in the fuel,oil,or the air system change the air filter and put some gas cleaner go to any auto part store you should be able to ask pretty cheap around 5 to 8 bucks hope this helped :)

Not sure what engine you have, most 4.8, 5.3, 6.0s are WAAAY back in the middle of the engine facing the drivers side, very difficult to get to.