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.. 
 
The monitors are called "non-continuous monitor tests", and the specific 
tests are: 
** Catalyst 
**Evaporative system 
** Secondary air system 
** Oxygen sensor 
** )xygen sensor heater 
 
According to the technician at the dealership, there are 5 preconditions 
("cycle flags") for these tests 
1. RPM below 2000 
2. RPM between 2000 and 3000 
3. Tank vent (didn't explain this one) 
4. time - 20 to 25 minutes 
5. another time 20 to 25 minutes 
 
 He suggested that one MIGHT be able to set the cycle flags by:driving in second gear, on a long level road, as follows: 
1. 40 sec at 1700 RPM 
2. 1:20 at 2000 RPM 
3. 1:20 at 2400 RPM 
4. 1:20 at 2600 RPM 
5. 1:20 at 2800 RPM 
6. 1:20 at 2900 RPM 

then drive for 22 minutes and then shut the car off and let it cool 
overnight, then repeat 
 
But, he said that it would be nearly impossible because if you drift off the magic RPM the counter starts over and you don't know it. After messing around for a while I decided (particularly since this was caused by a body shop and hence would be paid by insurance) to have the dealer fix it - they charge 3 hours labor (or more) to do this, so a minimum of $400.