I have a 98 El Dorado that a couple of years ago gave this trouble code. They changed the fluid in the transmission and erased the code. 5 months later and ab out 5000 miles it did the c ode again. We erased it and 5 months about the same mileage it came again. I was told it probably had a weak sensor in the transmission just to erase it. It happened a fourth time. Well, 2 weeks ago, same period of time and about same mileage it came again. We erased it but this time it came in 5 days and 180 miles. It has 68,000 miles on it it is a Northstar.
Trans needs looked at, dont wait unless you dont have the money, you could cause more damage, the input speed sensor is bad, and a couple of other things need looked at, seal on the input rod to the pump and the pump itself.