No trouble codes. Less odor with vents on recirculation. The fumes are present even while driving, so I lean toward the leak being in the engine area or forward part of the exhaust. There's no evidence of a rich burn, though, per my wife, the gas mileage has diminished. Otherwise, the car runs great (gutsy little engine). Visual inspection was negative both hot and cold, running or off. No black marks or obvious leaks. The engine has no obvious leaks, off or running. To my knowledge, the car doesn't have an EGR system. This was a big surprise. I thought it might be a slam dunk: a leaky EGR pipe. Apparently some Hondas had an EGR plate with a gasket and passages which commonly plugged. I thought there may be a leak at this gasket, but I don't see this setup on the car (2.4L). Pressurized the cold exhaust system with shop vac, sprayed soap. Found small leak at rear O2 sensor (Bosch universal replaced by me last year). Pulled it out, cleaned up the seat, sealed up nicely. The only area I couldn't inspect well was the manifold because it is blocked by heat shields. I also couldn't really see on top of the system, but i may get out the inspection mirror and check. I suspected the flex joint between the manifold and converter pipe, but here was no bubbling. I did spray the entire system, at least all that i could access. No leaks. Took it to a very reputable shop, they also found no leaks, but suspect it is the converter. Though I don't completely discount the possibility, it doesn't make sense that a bad converter will magically push exhaust into the cabin without an obvious leak. I voiced my concern with the shop, and they honestly agreed that the signs/symptoms don't align well. And they reiterated that they did not find any leaks. I can replace the cat and O2 sensors for around $500, but I'm not yet convinced it is the issue. I suppose I could troubleshoot the proximal sensor, but that still doesn't explain what appears to be an exhaust leak...and there is no code, and the car runs fine... I may run some more tests. It could be that the leak doesn't occur until the system is warm. I may warm it up and try spraying soap around, or block the pipe, or add seafoam and look for smoke (a nasty option). I'm wondering if I'm missing an emissions component. I haven't determined if the odor occurs only while driving (throttle open) or at all times. Maybe there is a not-obvious emissions component that opens with the throttle (PCV seems fine, opens with rev, but i could replace it--not easy, by the way). May take it to the Honda dealer: they know their cars, and the common issues. I'm going to keep looking, but wonder if you folks have any additional suggestions...? Many thanks, Bob<----need some clues!