Susan,
From what others have said concerning the 2511 code, the most likely culprit is the speed sensor. I found the info below:
The 2511 code relates to the output speed sensor on the rear cover of the transmission on the passenger side. First be sure it is plugged in correctly, then remove the connector and look to make sure the wires have not pulled loose from the sensor or backed up in the connector. It is also possible to have the engine speed sensor"The sensor at the front" and output speed sensor hooked up backwards. One person had a bad bearing in the transmission causing a problem with the speed sensor. The bad bearing produced some metal shavings, and since the sensor works on very close tolerances, the shavings interfered with its ability to read the speed of the transmission. I would check all the wiring and connectors first.