SHORT ANSWER
It seems that a TSA Pre-Check Authorization Code will not work on the TSA Pre-Check renewal website. However, my first hand experience did lead to the successful use of an Authorization Code for a TSA pre-check renewal, but that might not be repeatable by others without my specific circumstance: My pre-check status was about two years expired.
SUMMARY
I went through the process to renew my TSA precheck after it had expired about a year or two earlier. I first went to the TSA website to renew, where I got all the way through the process (very easily) and just had to pay. On the payment page I was offered a promo or "Authorization Code", and thought that it might be possible to use United Miles to pay for this. I found the MileagePlus website, which gave conflicting information about whether it would work for a renewal. I bought the code anyway, but it didn't work on the TSA website. I started the renewal process again, but it gave me slightly different pages to fill out then required me to go in for fingerprinting, and didn't ask for payment. While at the fingerprinting place, they were able to use the Authorization Code and I didn't have to pay. I received an email receipt with my UE ID and Authorization Code applied as a coupon, $78 paid. My KTN is the same. It seems like it will work for a renewal, if you are willing to go in for new fingerprinting. I don't know if this means that you have to let your pre-check status expire for the code to work. Someone else will have to risk a code purchase before their status expires.
STORY
It's post-COVID, and my travel schedule is picking up again, leading me to realize that my Pre-Check status expired in the middle of the pandemic. I went straight to the TSA site and clicked renew on the big banner at the top.
I went through the renewal process. It first asked for my KTN, then the typical "Where do you reside now", "Have you been there longer than 5 years", "Are you a citizen" questions. After that, it simply brought me to the payment page. I saw the field to add an "Authorization Code or Promotion Code". That made me think that maybe I could use miles to buy it. A quick search led me to https://securityfees.mileageplus.com/ where it very clearly says you can use the Authorization Code for renewals:
But after you sign in and click through to make the purchase, you are told that only first-time applicants are eligible and that by completing the purchase you agree to the TOS and everything else. This is clearly a case of bait-and-switch false advertising.
I went through with the purchase anyway. I figured I can always call and demand the miles back, which I find if you complain hard enough they will refund them. In light of this glaring contradiction between ads and execution, it's a pretty clear case to get refunded to make it right. I also thought that if the miles are that important to me, I can make a new sign up application and use the Authorization Code then.
After completing the purchase, it gave me the authorization code, and it also promised that the code could also be used for another person, not only me. I find it strange that usage is generous in this way but not the other. But maybe that's not true either, since we apparently can't trust them regarding how we can use the code. I'll find out though, since, coincidentally, I am using someone else's miles to purchase this code for myself.
I had the TSA renewal done and ready in another tab. All that was left was to pay. I immediately went to that page with my new Authorization Code, but was stymied with the following message:
Please correct the following errors:
This code does not exist.
I tried again after 10 minutes and 30 minutes and received the same error message. I decided to scrap the renewal and start over. I clicked the same renew button as originally and started by entering my KTN, but what was weird this time is that it asked for new information, like my height and weight, and also informed me I'd have to go in to show my identifying documents. Ok, but it didn't ask me to pay...
The Security Fees Mileage Plus FAQ restates that only first time applicants are eligible:
Who is eligible to use miles for the payment code?
Only members who are first time applicants of TSA PreCheck® and have a MileagePlus account address in the 50 United States, District of Columbia, Puerto Rico or Guam are eligible to use miles for a payment code. (TSA PreCheck® program membership is only open to U.S. citizens and lawful permanent residents.) Please visit our Terms and Conditions for more information.
Questions not answered by that FAQ are directed to the MileagePlus Service Center or the TSA. The lack of sense here is only slightly less offensive that the false advertising. It clearly says before clicking through that you can use miles for renewal, but then flips on you when you get to the final point. That the program offering is generous enough for you to use your miles on a different person, but not on anyone for renewal is strangely incongruent. Crap like this is why I won't fly United unless I really really have to. It's not just an apparent apathy for their customer, but an active antipathy and disregard. Ulgh.
A few days later I went to a local fingerprinting place that is contracted to work with the TSA for the pre-check needs. I brought my identifying documents and a printout of my Authorization Code receipt. The clerk wasted no time in asking for payment, but was just as quick in assuring me the Auth Code I had would work.
Happy ending! The Authorization Code worked and I received an email receipt with the Auth Code on it. No trouble at all. I have the same KTN and every reason to believe it will work when I travel again in a few weeks. I'll certainly be back to update the post if that's not what happens.
Safe travels.