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The 'N', 'O' or 'P' do not actually get transmitted from the diagnostic tool to the car, and neither is the last "checksum" character.
They are there simply to check that you entered the central 20 digits correctly.

If you want to you can select 'N' or 'O' instead of 'P', enter the next 20 digits very, very carefully, and double check them, and then commit.

You should be okay, but it's at your own risk.

Provided that you don't press the buttons on the fob two many times it doesn't really matter if it doesn't work.

so what could go wrong:-
If you press the fob buttons too many times then the rolling code "rolls" and the initial 22 digit code is no longer useful and so the fob is useless.
The Pektron has ten memory slots with two taken from the factory and so you have eight left to play with. Adding a new fob burns one of the eight. But hey you still have six left, and Technozen can clear them.

So I would say it's worth giving it a go.

I will add the 'P' in a future update.