I have a 2002 ford Windstar with 313,000 km on it and original oxygen sensors it recently passed an emissions test could changing the oxygen sensors increase fuel mileage, it averages 18-19 mpg or 12L/100km
Wondering if changing oxygen sensors can improve fuel consumption
The O2 sensors only report what is happening with the fuel / air mixture. If it continues to detect a too-lean condition, the computer will add fuel to all the cylinders. You have to identify where that extra air is coming from, not replace the part the measures it.
A vacuum leak, a misfiring cylinder, low fuel pressure, and an exhaust leak ahead of the catalytic converter will all introduce extra air but that should affect the emissions test. If the O2 sensors are switching from rich to lean a couple of times per second, they are working properly. Look for something else like dragging brakes, low tire pressure, etc.