Alright so I have been battling this issue with my cars ( 2003 Tahoe LT and Z71) for years now. I am writing this up to hopefully help someone else who is having similar issues with p1515 p1516 p0120 p2135 p0220 and all of the rest of the codes attributed to the fly-by-wire throttle system on these cars. I have taken care of all the grounding issues including the two behind the engine ( great idea GM engineers - hide the grounds in a nearly impossible location to reach ) and underneath the drivers side, I added additional ground wires, replaced throttle body and foot pedal sensor, replaced the 8 pin connector at the throttle body (https://www.ebay.com/itm/184415521710) and everything else if I missed something. The problem would go away and then come back with a different set of codes through out the years. I would do everything at once with the exception of the connector. Only mess with this if you are sure if there is a broken wire. But and here is the big BUT!...I have realized over the years that this system is very sensitive and while you messing with it, another wire could break. With that said, when if you decide to add a new connector and cut the wires, you need to make this bullet proof. I used shrinking/crimp connectors first. That failed and guys I am very careful when putting these connectors on. I used solder seal wire connectors the second time as the GM bulletin states you must solder these connections. Well this also failed. Finally I actually soldered them one by one. Not sure what I did wrong or right or why anything at the moment. But this finally solved the problem for the most recent reduced power issues I was having ( at startup and when I come to a stop at idle). Also, what may be important is to keep the fuse as short as possible and protect the soldered connections from heat. By fuse I mean the length of the wire to the throttle body. When you cut the wires, cut them longer on the engine side and shorter on the new connector in the case you have to do this again. But just be sure you make the length as short as possible or no longer than factory length. I think the sensitivity comes from the fact the the current running through these are so small. Also, when soldering and handling the 20 year old wiring, be extremely delicate. I believe this was my problem or the length of the wires were too long. It is possible you could break the wire inside the covering and not know it. Just remember that without any man handling of the original wiring, it broke on it's own. How much more chance of breaking a wire if you cut and bend and mess with the cluster! In any case I hope you can glean some info from this and solve your issue as well.