During cold weather the fuel consumption went up a lot. Diagnostics showed lambda sensor fault. Changed it, but fuel consumption is still 18-20l/100km. All diagnostics still say that it is lambda sensor fault, but I have already changed a couple of them

deville 87-4.1l

did you clear the codes after changing the sensor?

The code does not say to replace the sensor, it only tells you it has an Oxy sensor circuit fault. This code can be set by an engine running too cool, never getting into closed loop. It will set a rich condition Oxy sensor code when in fact, theres nothing wrong with the sensor and all it needs is a $10.00 thermostat. And running too cool will lower fuel mileage. The Oxy sensor can be monitored thru on board diagnostics to verify it swings correctly from rich to lean. You can also verify engine temp in celcius thru the same diagnostics. Should be 95 degrees C in about 10 min of driving. Hows the air filter? is the timing set to spec?

Air filter is changed, engine temperature is normal. Dashboard computer does not show lambda failure. Ignition timing is ok. Last time making CO tests all figures were OK at idle, but at 2500 rpm figures went out of normal and lambda showed 0,8.

OK, I'll bite. What the hell is a lambda sensor?

Thats the original terminology for an Oxy sensor.

Never heard that term before. Learn something new every day.