Hi all. new to the site. I have a 2002 blazer 4.3 with a p0155 bank 2 sensor one heater circuit code. I checked the resistance in the sensor and the heating element is good. I pulled the connector apart at the 8 pin connector on the right side by the a/c lines right side of motor. In looking at a electrical diagram, it shows a pink wire for battery voltage which I have and it is hot. However, this only goes to the post cat sensor. On this Eng, there are two green wires that go to sensor one. In checking wire integrity (ohmd) from the connector to the sensor all wires are intact. One green wire goes down into the loom to the Eng, and the other goes to the PCM. Checked from connector to PCM, wire is good. could not find where wire in loom to Eng goes. KOEOff neither wire is hot. put test light to both terminals with ground on pos post battey, and test light lights up on both terminals indicating both wires going to grnd. Tried KOENOn and no change. Does the pcm drive the power to that sensor and when it goes into closed loop, turn it off. If not, where would the power come from? I am Stumped. Thanks for any help.

 

The O2 sensor heater needs power & ground to function. Power comes from the O2 fuse in the underhood fuse box and ground should come from ground point G102 which is the rear of the right hand (passenger) cylinder head.

I'm not sure what connector you are talking about, but I do not see any green wires involved with the o2 sensors in the information that I have although my FSM materials are for 2000 & 2001 model years. If you have factory wiring diagrams for 2002, post up the applicable diagrams that show what you are describing and we'll go with your information for talking points.

You can also read through the P0155 DTC post for more information on what triggers this code. Further diagnosis can be accomplished with a scantool to monitor the O2 sensor output per the diagnostic data provided in the link.

 

I got the diagram from one of the other threads in a different area of this site. I got the car used and the previous owner did not give me much info or history on it. I do not have factory diagrams, however I pulled a lot of wiring out to try and chase wires and check them. The plug that I am referring to is a 8 pin connector r/s of engine that the 8 wires from pre and post cat sensors run to. This plug is located right in front of the heater box. The only problem is the heater. The sensor works fine when in closed loop mode. I have put a scan tool on the vehicle to verify proper operation.

 

Here you go. P0155 diagnostic chart and O2 Sensor Schematic for 2002 Blazer. Note the diagnostic chart includes a description of operation.

 

Based off the diagrams that Les provided, they did change the wiring for the later models concerning the upstream O2 sensor heaters in that they are now monitored & powered by the PCM rather than directly through the O2 fuse like the earlier 2nd gens were. Interesting.

You are basically going to have to go step-by-step through the diagnostic chart to get to the bottom of the issue. Don't skip anything and you should find the problem.

 

Many thanks to Les and all that helped with this problem. The diagram and info is exactly what is in my vehicle. I do appreciate it.