Thanks again! I'll check out the wiring around that junction tomorrow, and let you know what I find. I do still need to check that, but things have changed. I got a chance to check the resistance, and it's still showing an open, and the O2 sensor is reading 3.7 ohms across the heater. I now have 12 volts at the heater, with it lugged in, but I am still getting the P0155. Even though the sensor tests good with an resistance check, I'm still going to swap it with the other side, and see if the code follows. If not, I'm going to recheck all of the voltage and resistance measurements, providing the check of the wiring near that junction doesn't reveal any problems.