20 L / km is too high but you don't tell how you achieve this for the 400 to 500 km you're driving on the tank. All city, with 2 to 3 km trips where the engine doesn't warm up or a good highway stretch.

Regardless, as you have a clean air filter - the issue is quite likely related to engine management and the codes you see. Change your various 02 sensors (use Bosch) as well as your fuel filter (cheap prevention). Do you know how old your spark plugs might be? Change them ....

Then consider proactive (not so much even with the km you have) changing of transfer case, transmission and both diff fluids. Won't make a L / km difference but it will make the critical components last longer.