Air fuel ratio of 4!


Sep 14, 2021
Cumbria UK
POLO 1.2
My OBD Sensor reading (using a one from 'Torque') displays a value when the car is running and fully warmed up of around 3 to 4 :1 which is very low considering that the air fuel ratio of diesels should be around 14.7:1 to 15:1 . The 1.2tdi polo 6R is standard and has no smoke and runs ok apart from a little roughness between 1 and 2 thousand RPM and fuel consumption is around 50mpg which is a bit low for a bluemotion model. I suspect the Oxygen sensor is faulty...any ideas?


Veteran Member
Mar 3, 2014
2011 Jetta Sportwagen, 6M, red/tan, navi, pano, 83 5m diesel pickup, 82 p/u trailer,.04 5.5 TDI Passat wagon (gone), 80,81,82 diesel p/u (gone), 80,82 sportruck (gone), 59 passthru bus (long gone), 79&87 westy (gone), 57 baja bug (long gone), 73 914
14.7 @ sea level is gasoline engine mix sweat spot.

Diesel still baffles me as it seems so much more adaptable/flexible. I'm not sure if diesel can be held to many of the same standards as gasoline. Its a ability to make more power by cramming more fuel into the mix is certainly different than gasolines ability.

FWIW I also noticed the same 4:1 ratio at different times with my POLAR F.I.S. My thinking is it must be normal operation or the soot load would be very high.