Thanks for all your suggestions.
1. How do you test whether the resistor ends or the 02 sensor is malfunctioning? I do have the Sears multimeter that's famous in this forum but don't know where to set it.
2. And if the resistor ends are the problem, how do I change them? Do they screw on or are they plugged in?
3. Can one just change one resistor tip (in case only one is not working) or do all six have to be changed?