It’s a non-return valve, also known as a check valve, used in the cylinder head ventilation system, specifically for recirculating crankcase gases (PCV system). It allows airflow in one direction only, preventing reverse flow that could disrupt engine pressure or draw contaminants into the intake.
Fits
Audi A3 Hatchback (8P1) 05.2003 – 08.2012
Audi A3 Sportback (8PA) 09.2004 – 03.2013
Audi A3 Convertible (8P7) 04.2008 – 05.2013
Audi A1 Hatchback (8X1, 8XK) 05.2010 – 10.2018
Audi A1 Sportback (8XA, 8XK) 09.2011 – 10.2018
Volkswagen VW Caddy III Van (2KA, 2KH, 2CA, 2CH) 03.2004 – 05.2015