I had Tesla Mobile Service renew my cabin and HEPA filters on 2018 S in March only to find smell of occasional exhaust increasing. Around 60% greater airflow but persistent enough to make me suspect the cabin carbon filter part. So bought myself a Mahle LAO1613 and swapped them over, finding that service agent had incorrectly installed the earlier replacement, putting it in with the airflow marker upwards instead of downwards. Confirmed the air flow with HVAC on full fan and some paper inserted into the gap, sucked downwards. Also the listed part cost on invoice was just £7.49 for TESLA branded 107273600-B. Asked Service to resolve it after, the same part was listed as £20 - suggesting supplier troubles. Curious, but now cabin is odour free again.