A month ago I started getting a gas/exhaust fume smell in the cabin every time I turned on the heat (especially under 30 degrees) and was drawing fresh air. Searching this forum sent me into a panic after reading horror stories of blown spark plugs and faulty coils.

I brought it to the shop and had my mechanic search the car for any possible fuel or exhaust leaks. He also checked and replaced the spark plugs. One was looser than the others but not to the degree others were describing. No soot on the coils. After not being able to figure it out he sent to another shop that specializes in Hondas.

The owner of that shop laughed at me suggesting it was a "gas smell," but did admit the HVAC had a weird smell. He tried to convince me it was a dead animal and told me to clean the vents.

I couldn't use the heat on fresh air because every time I stopped or was inching a long in traffic the there would be a fumey smell. I was getting exasperated and decided to look around the hood where it draws fresh air in. There wasn't any strong smell under the hood when running the car or any obvious leaks. I found that the cowl was a bit loose as well as the weather stripping between the firewall and cowl was off the lip. I made sure the cowl was properly in place and secured the weather stripping. Now there's absolutely no smell from the vents.

Just an FYI for folks experiencing a similar problem. Worthwhile to check the plugs given the problem with this model year, but it might be something extremely simple like above. I'm beside myself that two shops didn't even consider this.