SOLVED - Upgrade MacBook Air ealry 2015 firmware problem

I created an offline USB installer to upgrade several machines (as in 15-ish). The first several machines updated without problem. I then tried to update a MBA early 2015. It would stop early on with "A required firmware update could not be installed". This is a stock machine with an Apple 500GB SSD (original).

I spent several days down the upgrade path including:

• do the upgrade from Big Sur to Monterey - fail
• erase the disk and do Monterey install - fail
• erase disk, install fresh BS then Monterey install - fail
• erase disk, use OpenCore Legacy Patcher to patch the boot EFI, install Monterey - fail
• erase disk (still has OpenCore), install fresh BS then Monterey install - fail

I spent 59 minutes on the phone with Apple support including the time with senior (tier 2) support. No joy. They did not know why / could not fix the problem. Suggested bringing into a store (Genius Bar).

While racking my brain, it occurred to me to try and zap the PRAM (command-option-P-R just after power on) might help (an often needed trick in the System 5/6/7/8/9 days). Never had the need in the macOS X days. Well, that did the trick. No problem with the upgrade.

Cheers,

eb