Some of you may have already experienced the dreaded readiness code failure while trying to get a SMOG certificate for your beloved 993. Below are some information contributed by: william_b_noble that may shed some light on your situation. Personally I had the same issue, I was able to resolve it by using a factory Hammer unit and multiple drives according the predefined conditions.
Below is the message from William:
It seems that the computer memory is held up by the battery, so if the battery goes dead, the computer forgets all the codes it's learned - that would seem harmless, but.... to pass pollution tests here in Calif, all the manufactures tests must have run. There are 5 of these monitors, and I can see them with my cheapie Inova OBDII scanner, but after several thousand miles, they did not set even though the car is running properly and emissions are well below the maximum allowed.
So, I spent some time with the dealer trying
to figure out how to set them. The upshot of all of this is that
without the special Bosh 9288 "Hammer" scanner, it is nearly impossible.
Apparently the 9288 shows the cycle flags, but other scanners that are
just OBDII scanners do not show them..
then drive for 22 minutes and then shut
the car off and let it cool