What can cause a Honda transmission to go into limp mode?

My 2004 honda crv works fine until its warm then it won't shift above second gear. What are your experiences with this? Is the internal filter clogged?|||Usually the Malfunction Indicator Light (MIL) turns on with a trouble code if the transmission is in 'limp mode'. Is that light on? And if 'yes' then get the code checked and report back.