Expert: Don Berlan - 7/28/2006

Question
Hi Don,
As part of my preventive maintenance, my oxygen sensor for my 1997 318i is due. I need to know (as I'm always asked) if my O2 sensor is before or after the catalytic? Thanks.  

Answer
There are o2 sensors before and after the catalytic converter.Is there a problem with either? I would have them checked out and not just because, as they aren't cheap.If your car is running great drive it,the normal maintenance items I would spend time on are filters(air and fuel),spark plugs,clean throttle plates and idle control valves,and a look over of all the hoses to make sure they aren't hasd and not cracked.
Under the car looking for any oil leak.Its important not to have any.A leaky seal causes havac with the plugs and the o2 sensors.That is why it is important to see if they work rather than just replacing them as you may have a problem elsewhere and the o2 covers it up or does nothing at all.