Service Engine light but NO codes

Hi there, my GF has a '92 buick century and her SES light and high temp lights blink when she's driving. This happens when the car has been running atleast 20 minutes, but happens at different speeds, on the highway.. and it also came on today while entering a parking lot. I scanned for codes using the 'paper-clip' method and all I get is the '12 code' repeating.. nothing else. What's a guy to do? the SES light comes on, so it was my understanding that it would automatically set a code. :\ Thanks for any help.
-Eric
email is ebentham@buffalo.edu or you can post. Thanks everyone!

It is possible that the code 12 is the result of loss of the rpm signal. Many people confuse this code as the all clear signal, when its actual meaning is that there is no rpm signal, normal with the key on engine off. Bear in mind also that a true high temp reading will illuminate both lights since coolant temp is a monitored parameter and excessive readings will activate both. I would research the possibility of engine temps rising to extremes. These would include, but are not limited to insufficient coolant, fan issues or other air flow problems related to this system. I would look into the possibility of a radiator fan problem. Ray

Hi there, thank you for the reply. I also went to GM and they scanned for codes... nothing.
The SES light blinks and SOMETIMES the temp. light blinks. The coolant is full, the fans are fully operationable, and the car isn't overheating at all. Her father swears that it is the crankshaft sensor. and, coincidently, I did read somewhere that these problems would happen, [hesitation,chugging,NO code would be set]. I've been meaning to replace it, however, she has a 3.3 6 cyl. and it's my undersanding that it is buried.. Also, if you don't get it just right, it will destroy the sensor.. so I went to a couple of shops to check on pricing.. It's going to cost about $150-200. Gladly, I would pay this...if it Fixed the problem .. but.. who knows :)
Sorry for the long post, just in a debacle here.
-Eric

The key here is testing. If the shop in question understands the operation of the system, it can properly test it. If not, keep looking. There is no need for guessing. Ray