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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.

EDIT

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

  • Hook it up to a scan tool and report back what you're seeing for long-term and short-term fuel trim values – Zaid Mar 24 '15 at 11:43
  • @Zaid The car is not OBDII compliant, so all I can see when hooking up are current values of various sensors and if there are codes or not. – Robert S. Barnes Mar 24 '15 at 15:22
  • Do you have O2 sensor voltage(s)? Lambda is another name – Zaid Mar 24 '15 at 18:42
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Disconnect your battery to reset your ECU. It won't hurt, and is probably the cheapest and easiest thing you can try. 30 minutes is usually enough.

There are other theories as to why your mileage went down, but I think you're on the right track thinking the ECU hasn't learned to use the new data correctly.

I am thinking that because your new 02 sensor is clean, it reports a higher (more accurate) O2 reading, which tells your ECU to inject more fuel than usual, making you run richer. Which isn't a bad thing in terms of engine longevity, but performance and consumption will suffer, as is your case.

Or if it's a different brand, you may need to have your new O2 sensor calibrated. It will require re-flashing your engine management system, so if that's going to cost more than what you expect to save from improved mileage, it may not be worth it. You can expect to pay between $200 and $600 for the exercise. This basically involves having a second, wideband O2 sensor temporarily installed alongside yours and fiddling with ECU lookup tables until both sensors report the same thing.

  • Sound interesting, I'll try it out. – Robert S. Barnes Nov 18 '14 at 7:51
  • It's quick and free ;) – Captain Kenpachi Nov 18 '14 at 8:16
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    It won't hurt, but I'll be surprised if it's the issue. OBD-2 ECUs should have both short and long term fuel trims updated within 15 minutes of driving even without a reset. – Brian Knoblauch Nov 18 '14 at 17:55
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    @Brianknoblauch It's a Middle Eastern model and probably not completely obdii compliant. – Robert S. Barnes Nov 18 '14 at 17:59
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    That improved mileage was a one time blip, probably due to the injector cleaner I put in the tank and driving almost completely highway at 500 meters below sea level ( I usually drive about 1000 meters above sea level in mountain terrain ). Since that one blip, I've had consistently worse MPG's than before I changed the O2 sensor. – Robert S. Barnes Mar 23 '15 at 19:16

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