[bostogrun]

Don't mess with them unless you have a CEL code. Sometimes, they dont come out so easy especially if they have been in there for years. If you trash a sensor trying to get it out you go beyond the point of no return. Then you will be removing the whole exhaust system or even worse an exhaust manifold trying to get it out. Make sure that you use a o2 socket. It can be a real b_____h getting just the right access and leverage to the two in the exhaust manifold. For some reason 02 sensors have a soft hex unlike a spark plug and round off super easy. Once the hex is rounded your going to have to remove exhaust components. Many exhaust shops consider o2 sensors unremovable.

BMW and many vendors recommend changing them after 100k as preventative maintenance or as part of a "tune up" If a car has been well maintained ( in this case meaning frequent air filter changes) the sensors can last the life of the car. The sensors are actually voltage generators develop a wave form usually between .2 and .9 volts. The CPU monitors the sensor output and adjusts the fuel supply to injectors. The CPU is pretty darn good, almost too sensitive, regarding detecting problems with sensors. Many times they trigger a CEL with no driveability issues because the CPU is so sensitive. Usually, a rich condion is associated with bad sensors. You will notice poor fuel economy, rough idle, the smell of gas under the hood and diluted oil. The O2 sensors don't have any thing to do with starting because they don't come into play untill after the car is in "closed loop" i.e. fully warmed up/