Engine light came on my 99 Accord, the diagnostic codes P0700 and P0740 came up. Whats the meaning?

 

P0700 = Is a generic trouble code that indicates that the transmission control module (TCM) has detected an issue within the transmission control system and requires further diagnostics. This is why you also see the P0740, that is the more specific fault that has also been reported to the ECU from the TCM.

P0740 = Transmission lock up control fault
Possible causes
- Low transmission fluid level
- Dirty transmission fluid
- Faulty torque converter clutch solenoid valve
- Torque converter clutch solenoid valve harness is open or shorted
- Torque converter clutch solenoid valve circuit poor electrical connection

 

You will need to verify correct operating pressures and flow through the cooler.
If this is an original trans, it may be the PRV is faulty causing the converter clutch to drag.
A few articles on the problem
Sonnax Converter Charge Regulated Pressure Regulator Valve - 98892-04K