Klaus,

As always, thank you for your response. I will inquire about the specific code(s) that triggered the CEL. I have replaced the O2 sensors on this car from time to time. My Volvo is equipped with two sensors: front and rear. I can't recall off hand how long ago a sensor was replaced last. I always have two O2 sensors in reserve so when the CEL is triggered, I leave them in my Volvo so my private garage can use them, instead of billing me, should an O2 sensor be the cause of the CEL.. I only buy Bosch O2 sensors on line. My usual practice is, when I replace an O2 Sensor, I immediately purchase an O2 Sensor for the one that was used. When I receive the replacement, I immediately write the date of receipt on the box. I don't have the two O2 boxes in front of me now (my Volvo is still in the shop), but I seem to recall one sensor was purchased over a year ago, and the other was purchased 2 years ago. I can't recall if they were front or rear. My understanding is, the front O2 sensor that screws into the exhaust manifold has a shorter life span than the rear O2 sensor which is further down stream, I think after the catalytic converter?

I will mention the O2 spacer to my mechanic. A suggestion by James below has given me hope. Just before the CEL came on I was about to replace the spark plugs. I replace the spark plugs every 30,000 miles. I ordered them from iPd but they have not arrived yet. My plan is to install them and ask my garage to reset the CEL light. My hope is the new Volvo plugs (that is all I use) will fix the problem. I guess from my lips to God's ears.

Hugh