991 gas smell when coming to a stop

[Benjy911]

I'm getting a natural gas smell (like turning the hob on but not lighting it), that appears for a few seconds when I've come to a stop. If I put the recirculation on it's not happening.

I recently replaced the battery and hadn't noticed it before that, so I've put the old one back in but now can't work out if I'm imagining the smell or not! I don't suppose anyone else has had this and worked out where it's coming from?

[hardlylikely]

Probably your catalytic converters are failing, normally described as a smell of rotten eggs. Mine do the same, but still passes the MOT ok, perhaps give it a good thrashing might help

[Benjy911]

I wondered about the cats but I'm not getting the smell when cold and I think I'd notice it when getting it in/out the garage. It's only done 39k miles, 2k of which were done in the last 10 days blasting through France!

I've also only noticed the smell with the roof up, which makes me think it's coming from the front near the air intake.