My two cents.Ya check you fluid Next see what code it is first some time a pressure switch,clutch pressure controls solenoid or shift solenoid go bad. You could also just have a short or an open in some wiring.

If the code is a tork converter lock up solenoid or incorrect gear ratio code kiss your trans good bye or have it rebuilt.

You can also do a stall test after checking the fluid and see what clutch is slipping. How do you do a stall test you say put it in 1st hold the brake smash the gas you engine should not rev beyond 2000 rpm next put it in 2nd hold the brake smash the gas again 2000 rpm next 3rd 4th and r you get the point.

Lets say in 1st it revs to 5000 rpm and 2nd to 1600 rpm. 1st clutch pack is shot or not getting pressure.Hell start it in 2nd and shift to drive later.

Now some time you can call your district honda rep and get a free trans replacement all the numbers are in the back of the owners manual. Make up some sob story about how you only have one leg and you have cancer and molest the time it workers. I have done timing belt under warranty before.