All of a sudden I got the message.
The tank is 3/4 full and the cap is fully tightened.
Any ideas what is causing it or how to reset it?

2013 MDX

 

I had the same problem with my 07 tl-s. I did some research and found out it was the purge control solenoid. Changed it out and haven't seen the message since. Not sure if it would be the same fix for your rdx though.

 

^^^oops meant mdx

 

I got the same message twice, I open the fuel cap and retighten, I usually do 3 clicks, but decided to do 5 clicks and message went away. Few days later came back on and repeated the process and it hasnt came back ever since.

 

My 09 did this a few times and there was never a reason for it. Always went away with time.

 

Tighten is by twisting it until you hear the clicks. It'll reset on it's own after a few miles.