https://www.f150forum.com/f38/p0299-resolved-389774/#post5409034

above is the link to my post from July of last year when I first got this code and they replaced both turbo assemblies and other related parts. The code came back a few months ago but with no driving changes to the truck so I cleared it and gave it a few days to see what it would do, it didn't come back for another month or two and that's when I called my service advisor. He told me to bring it in to have it looked at and they were able to pull the same code off of the truck and start diagnosing. They had it for about a month but I think only work on it for about the last week because they were backed up. In the end it turned out to be a valve / seal / solenoid assembly that runs between the intercooler and the Turbo part number aa5z9u465a. Attached are some pictures of my repair invoice in case anybody wants to see the testing they did to come up with this fix. I did forget to ask my service advisor when I picked up the truck yesterday if he thought that maybe this was the problem back from last year that might have been misdiagnosed but it came to me this morning that last year they ran tests on the turbos and saw that there was a problem with the wastegate switch they had an open TSB for which is why they replaced both turbos assemblies. Again that my truck is under warranty so I paid $100 for the repair and the invoice doesn't show a billable amount so I don't know how much this will cost. But you can't see on the last picture an estimate of how much mechanic time under M time was claimed by the mechanic for running all of the tests and doing the repair so that kind of gives you an idea of what it might run I'm guessing that the times listed on the third image under emtime claimed, are in minutes. I hope this helps someone down the line if they have similar issues.
just to let everyone know I did not have any change in characteristics when the code came up the truck felt fine I do use it to tow up a grade with the boat trailer and quads and other stuff and never had an issue as far as loss of power or that the truck felt off I just have to like come on. But I will say that it does feel more peppy than it did before this repair.

 

Thanks for posting this. I just bought a 2013 FX4 a couple weeks ago with 54k miles. I just had the P0299 code pop up. The previous owner gave me all of his service records, one of which was a turbo boost sensor replaced about 4k miles ago. I got in touch with him and he was not aware of this code, and the service record confirms that it was a different code for the sensor. Nice guy, offered to take the truck in for me in case they wouldn't honor a potentially related service for me as the new owner. Anyways, with the Underboost code, I did not feel any change in performance, and it seemed to come up just cruising on the highway at 66 mph. I cleared the code, hoping that 1. it never comes back, or 2. it comes back within the next 5k miles before my warranty expires. Going to save your post in case I do run into it again.

 

My problem has been resolved and seems to have been very similar. Except mine was completed and back to me in the same day. They replaced a wastegate solenoid (covered by warranty) and a turbo bypass valve (not covered by warranty). Code is gone and the truck seems more responsive when applying power now. Total cost was $181 for me. $75 part with $120 labor and the guy gave me a discount because he misquoted me on the phone early in the day but still honored that number.