My daughters 2001 Honda Civic has a P0740 code as well but there is no difference when driving it. We have reset the code 2 times and it always comes back on once she goes over 45.

 

P0740: Lock-up control system (auto trans)

If there are anymore codes diagnose them first.

How long has it been since the last transmission service (fluid change)? Does the transmission fluid look dirty? Does it smell burnt?

Drain the fluid through a strainer. Is there any debris? If there's debris you're trans is going and you will probably need a new one soon.

To go any farther you need a pressure gauge and adapter for the transmission or you could just replace the solenoids but, that's expensive ($400) and you may just be throwing money away if it's the whole trans going. If you do have to replace the trans go with a 2004-2005 as they went with better clutch materials and pretty much fixed the problems these trans were having.

 

need to replace transmission assembly unfortunately. do not replace solenoids or perform any transmission flushes, it wont fix it. the only way is to replace the transmission assembly.