My 2005 tahoe is going into reduced engine power. i have replaced the throttle assembly and the harness. this seemed to fix it for a while. now it does it when i hit a bump. After talking to a GM tech i have deduced the problem to a black box mounted on the driver side firewall. it has two harnesses plugged into it. when it goes into reduced mode i can turn it off, wiggle the harness and it seems to act fine for a while. my question is, what is the name for this box and is there anything i can try without replacing the whole setup. Please help, i am going crazy over this

Problem with electronic throttle control . search reduced engine power on youtube . Have the vehicle checked for DTC'S - diagnostic trouble codes . And don't listen to advice on forums ,they have no clue . It's probably a TPS - throttle position sensor or one of the sensors in the APP
The APP sensor is fixed to the throttle body of the electronic throttle control system with intelligence (ETCSi). The functional principle to the AAP sensor involves conversion of the accelerator pedal movement into a voltage signal.
Inside a Car - Accelerator Pedal Position Sensor vs ... I would tell you to take it to a qualified repair shop but most people don't want to hear that . Untill it's too late .

Pretty much, the throttle body actuator is the small motor that opens and shuts the throttle blade and is serviced in the housing assembly.

I've had the same problem with the reduced power and tps sensor which is your gas pedal. I replaced it but the problem still existed. I then changed the throttle body and harness to the throttle body but still had the same problem. So I traded the vehicle. I did everything the dealer suggested and I just cut my losses. The Yukon overheating would be a different problem and should be easy to fix.

P0894 Transmission Slipping
P1258 Engine got Hot
P1516 Throttle Actuator Control (TAC)

Chevrolet Tahoe vehicles may experience a performance issue where they will not go over 15MPH. The vehicle may appear to be in "limp-in" mode, giving just enough power to get you to a shop. The codes indicate a problem with the throttle position sensor circuits and throttle actuator performance. Do NOT start replacing parts. Shops report a pattern failure in the electrical harness to these sensors. Chevrolet has issued a revised replacement harness to correct the problem. The original connector was prone to vibration and broken wires that disrupted the signal to the PCM and put the vehicle into reduced power mode.

There is a TSB update that is for your vehicle TSB Bulletin No. 05-06-04-032C. Tittle Engine control - Low power SES/lamp on. What they want you to do is replace throttle body assemble and harness connector to throttle body. Now hopefully your problem may just be a connection intermittent failure to throttle body itself.

The check engine light being on now is a different problem , Have DTC'S - diagnostic trouble codes checked ,will have something to do with electronic throttle control . That is what reduced engine power is about . It's in a reduced power mode.
Reduced Engine Power Mode
When the ECM detects a condition with the TAC system, the ECM may enter a reduced engine power mode. Reduced engine power may cause one or more of the following conditions:
• Acceleration limiting--The ECM will continue to use the accelerator pedal for throttle control; however, the vehicle acceleration is limited.
• Limited throttle mode--The ECM will continue to use the accelerator pedal for throttle control; however, the maximum throttle opening is limited.
• Throttle default mode--The ECM will turn off the throttle actuator motor and the throttle will return to the spring loaded default position.
• Forced idle mode--The ECM will perform the following actions:
- Limit engine speed to idle by positioning the throttle position, or by controlling the fuel and spark if the throttle is turned off.
- Ignore the accelerator pedal input.
• Engine shutdown mode--The ECM will disable fuel and de-energize the throttle actuator.

Do you know what electronic throttle control is ?
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Throttle Actuator Control (TAC) System Description

The throttle actuator control (TAC) system delivers improved throttle response and greater reliability and eliminates the need for mechanical cable. The TAC system performs the following functions:
• Accelerator pedal position sensing
• Throttle positioning to meet driver and engine demands
• Throttle position sensing
• Internal diagnostics
• Cruise control functions
• Manage TAC electrical power consumption
The TAC system includes the following components:
• The accelerator pedal position (APP) sensors
• The throttle body assembly
• The engine control module (ECM)

Your best bet , take it to a qualified repair shop .

I have a 2008 Tahoe that had a similar error stabilization off, ABS errors, entire vehicle shutting off, door locks open/close at will, I changed out the Body control module and the programmer showed me the many communication errors from various modules, then one solution was suggested to change the Battery cable, so I started checking out the grounds and I found varying resistance from the Neg. Battery post to the Body so I removed the neg. cable to the engine and replaced the post with a threaded post connector and a 42 inch cable then added a 12inch to the frame, then also added a 12inch from the Neg. post to the body just under the Mega fuse, Make sure the paint is removed before you assemble and use the appropriate amount of washers to make the mega housing even to the body. No more errors, it was just a real bad ground and GM knows about this and it seems they are using this to make a lot of money off there unsuspecting customers,

yeah when the reduced power engine light comes on that will happen. that will happen for a couple of reasons. use a code scanner and it will tell you what exactly is wrong with the car and you can clear the code when your done.

it sounds me like the app sensor is bad, i dont think tps will cause reduced engine pewer, but app sensor will . u have to replace the trottle body to fix the app sensors uder the hood.
replace the accelleator pedal to fix the app sensors n the cab.
there is a ground wire in the big wire loom behind the intake if u loose ground ull get that reduced power issue i replaced throttle body and accellerator pedle n had the same reduced power few minutes after test drive came back cleared the codes then found a hit on idenafix for ground in the big wire loom ground went to fire wall and was bearly hanging on broke when i touched it fixed it witha crimp on fitting, havent had the code set since

you have a problem with your throttle body..have dealer read codes and repair...could just need cleaning or have a wire broke in the harness