4km per litre?
Yikes.That's really bad, looks like your mixture is way off.
Your O2 sensor may be bad but I have yet to see a post that reports that bad a mileage from a faulty O2 sensor.
How does the car perform?
Smokes?Hesitates severely from acceleration?Heavy exhaust smell?I'm curious.Is the car driveable at all?
Ctaylor is right.Get the mixture sorted out.
If you are a DIYer, disconnect the EHA and see if the car performs better.
If it does, it is more likely a sensor that has gone bad eg O2 sensor, EHA, etc.
If it is the same, then one of the mechanical components may have gone bad eg fuel pressure regulator, fuel distributor, etc.
With the EHA off, the car's mixture can be tuned to run mechanically well, independent of the sensors.
The only problem you will have is cold hardstarts coz with the EHA off you get no cold start enrichment.