Check engine light came on at stop light last wk-107K miles on car. 2 different AutoZones said codes point to front oxygen sensor. Bought a Bosch unit from them and installed, disconnected battery for 1 hr and upon restart, light still on. Called a friend who has Volvo repair shop and he proceded to tell me that vacuum line on intake manifold maybe blocked--this is common and will show code that its an oxygen sensor. I checked the one he mentioned,its located just under the thermostat going to a black tower/vacuum junction-sorry dont know proper name. Junction is under the black plastic cover where the throttle linkage connects to the throttle body. It was OK, BUT the line from the PCV valve to this same junciton was completed blocked and brittle. I tried to pull apart and it broke in three pieces. Replaced the line with rubber line and started, check engine light out! Bottom line, check these two areas FIRST if you get a check eng light for front O2 sensor first. I could have saved $151......