o2 sensor

[el_gato]

I have an 04 M3 with 116,000 kms. My check engine light came on and it turns out i need rear o2 sensor. The part itsefl cost like approx $280 from the dealership. Getting to the point anyone here has to get this done. And does it make a huge difference I just leave it as it is i.e fuel economy.

[FLIPDADY]

You can also add damage to your cat-converter as well.

[el_gato]

thanks for the input guys...

[igzy]

Just get a universal one for $60, do some soldering and call it a day :)

[dentinger]

i ran without an O2 sensor when i installed my header. i just got pretty shitty fuel economy, and a CEL.

[igzy]

Front O2 can affect you A/F mixture but rear O2 only monitors it, i.e. should not affect your gas mileage...

One can also use a simulator if it is the sensing line problem (signal), if it is heater you may need a heater simulator or replacing the sensor altogether (same for front if you did the header replacement)...

[el_gato]

so leaving it alone whould pose any major problem? the check engine light annoys the hell out of me though. so far my fuel economy stays the same. around 440 kms per tank. i fill up as soon as the gas light comes on. and its a 2.3 4 speed auto.

[igzy]

so leaving it alone whould pose any major problem? the check engine light annoys the hell out of me though. so far my fuel economy stays the same. around 440 kms per tank. i fill up as soon as the gas light comes on. and its a 2.3 4 speed auto.

Is it the rear (after main cat) O2 that comes up? If it is, then it shouldn't cause you any gas mileage probs...

Your consumption depends on many things like driving habbits, city/hwy, tire inflation, tune up, etc...

[Cosmo77]

actually if the rear 02 sensor is reading improperly or not at all.....it can also create a fuel milage problem. Yes the front 02 sensor monitors the 02 levels before the cat, but if the rear 02 (the rear 02 monitors the cat efficency and is used for fuel trim mapping purposes) is stuck lean or rich....well then you can all sorts of fuel milage or potentialy catalist damaging conditions.

Its best to just get it fixed...

Cosmo

[igzy]

In typical set up, front O2s monitor a/f mix (hence called A/F sensors) and will adjust it according to exhaust composition. Rear O2s monitor cat efficiency and oxygen levels (hence called oxygen sensors) after it and do not adjust a/f mix (passive). So if rear throws a code then it will not affect your fuel consumption but just tell you something is wrong. When front throws a code then the ECM falls into default level and will cause engine to run rich, i.e. kill your fuel mileage. This is traditional emissions and a/f control. I will confirm this for Mazda set up but typical engine (and I think Mazda's too) operates this way :)

[el_gato]

Lots of info thanks so much guys.

[igzy]

Thanks to help from a fellow TM3er I learnt that Mazda unlike other OEMs uses the rear O2 for A/F adjustment... So if your is out then your fuel mileage will be off too... This is so different than Nissan, Ford, GM and some other OEMs that use rear O2 to only monitor cat performance and front ones to adjust A/F...