You can get to it without removing the plastic air intake but it is a little tight.
Reach in to the left of the plastic air intake box. Mine had a little dirt on the opening,
cleaned it with a pin. (32,000 miles) I will clean it at every oil change from now on.
Need a bit of help.. even with all the pics I can't find that breather cap (COVID brain fog confirmed?). Using this picture of the engine bay, can someone circle approximately where that breather cap is located?
as viewed in this pic- the engine is "behind" this assmebly, and the transmission would be in "front".
The breather is on top between the two, and closer to the front of the car (nearer the radiator than the firewall)
SanPablo's post #24 in this thread with the pictures should allow everyone to find the HRV CVT trans vent plug.
Pull off the air intake tube if you are still have trouble finding it.
Once you have found it, you will be able to remove and re-install it easily just by feel without removing the air intake tube.
Soak the rubber vent in a mild solvent for a few minutes to clean it quickly and completely.
As an aside, a few months ago I discovered that the 6 speed manual trans in our 2006 Acura TSX has exactly same rubber vent cap. I have never noticed it nor cleaned it before but it was spotless.
Then I had a peek at the parts catalog for the 6 speed manual trans in an HRV.
Sure enough, the HRV manual trans has exactly the same rubber vent cap.
The manual trans folks should be checking and cleaning their trans vent cap also.
BTW, the trans vent caps have all the same p/n. 21396-P20-000
HRV CVT, HRV manual trans, 2006 TSX manual trans, Civic CVT plus likely many more.
Edit: Found the vent plug. Thanks for all the help!
For those seemingly challenged like myself, the valve cap is (from the Driver's perspective inside the cabin), is 45° to the front and right of the fill plug (the one with the hook). With the placement and position of the various tubes of this HRV, it is entirely obscured by view from above, either standing at the tire wheel or in front of the car. To locate it, stand in front of the car, look directly downwards and look for the bolt facing the starboard wheel circled red. The vent plug is just to the rear of that bolt, on the same elevation. Remove and replace by feel alone.
As for my vent plug, it was relatively clean. This company HRV is at 28,000km, driven entirely between farms. Enjoyed a blast of concentrated farm fertilizer smell cleaning the vent plug.
The vent plug is oriented to face the front however. Honda even marks it with a an arrow and "F" inscribed. This begs the question. Why?
Why would the vent plug be oriented to face forwards? Would not this only introduce ram air foreign debris into the transmission case?