A code P0401 most likely means one or more of the following has happened:
The DPFE (differential pressure feedback EGR) sensor is faulty and needs to be replaced
There is a blockage in the EGR (tube), most likely carbon buildup
The EGR valve is faulty
The EGR valve may not be opening due to a lack of vaccuum
this tool can do honda,you can have a look.
Thanks for your reply. Unfortunately, the 2004 CRV has no EGR. Per the previous person's advice, I asked Scotty Kilmer and he thinks it's probably some kind of software issue and that I should see the dealer...