I changed the O2 sensor on my 97 Mazda 323 BA 1.8L BP DOCH Lantis and my mileage actually went down. The car had the original sensor in it, with about 200,000 kilometers on it and I replaced it with an NGK O2 Sensor which I bought from the Mazda dealership. According to fuelly similar models from the same year where getting between 9.8 k/l and 12.6 k/l, while I was getting about 8.3 k/l. After changing the O2 is went down to 7.9 k/l under the same driving conditions. I've driven 247 kilometers since I changed it.
What gives? Do I need to reset the ECU maybe for it to adjust to the new sensor? This is exactly the opposite of what I expected based on everything I've read here and elsewhere.
It says on the autozone guide that turbocharged model need to have the negative battery cable removed before installation, but I'm not sure if that's relevant for my model.
So I reset my ECU, and my MPG's have still been consistently worse after changing the O2 sensor. I discovered that I had been driving with no thermostat in the car, but putting one in didn't make any difference in my average MPG's. I've also cleaned the throttle body and IAC without any affect on mpg.
According to the spec sheet, the car is supposed to have a combined fuel economy of 12.65 km / liter