In message , Dave Arquati
writes
Which brings me to a couple of questions. Firstly, on prepay, is the
full Z1-6 balance deducted from your card when you touch in and
credited back to the correct amount when you touch out, or does it just
reject your card at the exit gate if you have insufficient credit? Has
anyone tried this?

If you are in Zone 1 you must have at least £1.60 to enter the system
(£1.00 in other zones)
If you don't have more than the minimum for the zone and try to exit in
another zone, the exit gate will open but the POD will show Seek
Assistance and 24
You wouldn't be able to use the Oyster again until the Pre Pay had been
topped up and the debit cleared.

Secondly, why on earth are Carnets cheaper at £1.50/ticket than Oyster?
I thought TfL are meant to be encouraging Oyster. At the moment there
is no particular reason to persuade anyone to get prepay other than the
novelty - since Carnets are cheaper and prepay can't yet be used on
buses?

I don't know why either but it may be because you can't buy magnetic
tickets with Pre Pay as yet. When that comes in, maybe other things will
change too.
--
Kat Me, Ambivalent? Well, yes and no.