O2 Sensors

[lionbat]

My 2001 E46 330xi recently turned 74,000 original miles. The car is running very well but fuel economy has been worsening. I wonder whether the O2 sensors should be replaced as a matter of course (they are original)? What do you think?

[Manolito]

O2 sensors are a routine maintenance item. The recommended replacement interval is in the Emissions Warranty Information manual that comes with the car.

On the e36, BMW recommended replacement every 90k miles. I found that 105k miles was about as far as I could make them last on my e36s.

On the e46, BMW recommends oxygen sensor replacement at 120k miles. I have 140k on my 325iT, and the computer is complaining about the oxygen sensors. This car will get new sensors as soon as some other bills are paid. My fuel pump is failing, so the oxygen sensors will have to wait a little longer.

A bad batch of fuel can contaminate your oxygen sensors and cause premature failure.

A good shop can tell you if your sensors are bad, but don't bother trying to test oxygen sensors with a hand-held meter to see if they are OK. The hand-held meter will report that they still cycle and put out decent voltage long after they have gotten too slow to work well with your fuel injection and OBDII. An oscilloscope will show that the signal ramps up slowly on an old sensor, while a new sensor puts out a nearly square wave.

On an e36, the first indication of O2 sensor failure is "Catalyst Efficiency Below Specs" code. On my e46, it is actually storing non-fatal oxygen sensor codes.

For bad fuel economy, check the usual suspects like tire pressure. Air filter should not affect fuel economy unless it is really bad, but take a look. BMW says the original spark plugs should last 100k miles, but I don't run any plugs, even platinum or iridium, more than 50k miles.

Both of my e46s have now lost fuel pumps, and thinking back, the first symptom both times was poor fuel pressure at high fuel delivery rate, causing the car to surge and misfire on uphill grades. I track my fuel economy on the 325iT, and I did loose a couple of MPG when that problem started. Of course, if fuel delivery is suspected, first replace the fuel filter.
Fuel_Filter

[lionbat]

Thanks so much for a knowledgeable reply!!