If you mean a throttle booster as in one of those Vitesse Throttle Controllers or Pedal Max etc...then yea they are known to sometimes cause problems with the ECU. Sometimes it throws the readings haywire and the engine goes into "Reduced Power Mode" because it cannot correctly read throttle input or tell where the pedal actually is. If you have one of those throttle controllers and you go into limp mode, then simply remove it and the engine will operate normally although the CEL will remain until it is cleared. If it happens on a continual basis then you should just remove the controller completely and don't use it again. It is either defective or your ECU won't play nice with it. Either way it is not worth the fix and hassle.
As far as the warranty, if that is indeed the issue the dealership will not honor a warranty. Because technically it isn't the engine, it is the throttle body, ECU, and controller all having miscommunications. The ECU throws the engine into limp mode simply to save you...since it can tell where the pedal actually is, the ECU is protecting you and everyone around you from disaster. If it can't tell where the pedal is, then you could be lightly tapping the gas and it'll think you're flooring it which would be bad in traffic or a crowded street. So it goes limp out of safety. But again, it isn't the engine, it is the other factors i mentioned.
What I would recommend is getting a cheap scan tool. You can get them for like $150 for some really good ones. And they will save you a lot of trouble in the future. If you do have a throttle controller on then remove it to be safe. If it isn't a throttle controller then it is up to the mechanic to figure it out. It could be the electronics in the throttle body going bad or maybe the ECU is corrupt somehow. It could be a number of things. Keep us updated on how it works out. And we're always here to help. Sometimes we're a little slow to reply but we are here. I and others will help you along the way. I'll check back daily to see any updates.
2017 Camaro ZL-1 A10
2013 Jeep Grand Cherokee Hemi 5.7 - Tuned!!
Proud to be the March 2013 Member Of The Month!