I have a 2006 gmc sierra 1500 with a little bit of a newer engine I put in it about 3 years ago. Ever since then I been fighting with the engine going into "reduced engine power" mode with dtc p2135. I've done plenty of research and so far I changed the throttle body about 5 times(lifetime warrenty on throttle body), the peddle position sensor, checked and fixed grounds and plenty of other things that I can't remember right off the top of my head. After all that im still getting the "reduced engine power" message followed by the dtc p2135. After doing some of the mentioned fixes above it seemed to have help for about a month at least and then back to doing the same thing. Im getting very frustrated with it as I know its probably something simple but just don't know what to do next.

Welcome to the forums.
I assume you installed the 5.3 but what year is now installed?

I'm not sure what year it is cuz the company i got it from said they were only able to tell me its a little newer engine with less miles than the one i had. If I were to guess it may be an 07 cuz when I go buy an oil change some times I have to say its a 07 classic or I wouldn't get the right oil filter.

Was this a salvage yard engine? Find the engine block serial number and decode it, to understand what year you are actually dealing with.
The throttle bodies are problematic and fixing the P2135 is not as easy as one may think.
There were GM Technical, service bulletins, Repair kits, and PCM flashing & software updates etc etc..
Knowing the year is very important. What exact throttle body is installed a GM/ OEM or an aftermarket ?????
This video I found useful:

That tells me you got an unknown year and unknown mileage. They don't know the year but they know the miles?

Sounds like one of those yards that yank engines and tell everyone it has 46,000 miles on it no matter how many miles are really on it.

From your description I infer that the previous engine ran fine and the current engine never has run fine. I assume the previous engine ran fine and developed a knock or had a rod exit the block or somesuch. Overall this means the new engine itself is the problem . . . or possibly something that was disconnected from the old engine and not installed on the new engine properly is the problem.

I agree with the others. We need to know the actual year of the engine.

Sorry for the delay in replying back. All I found was that some numbers on the block and cylinder head. the numbers on the block are as follow. rear of block 12567392 and the front of the block passenger side head 19f06. Also the numbers on the heads are 706 and the oil pan is 12577396. I tried searching these numbers and found nothing on what year the engine is. I'm pretty sure its an 07 tho as the oil filter that fits the oil pan is that of an 07 classic. I still could be wrong tho. If somebody knows how or where to go to decode these numbers I would appreciate the info.

Try this: https://help.summitracing.com/app/an...aVElMjElMjE%3D