I am called to Mr. H's shop for a full day of diagnostics. In fact he has so many for me this day I get my own parking lot. I feel special. One of the cars is a 1996 Ford Contour 2.0L with 63,146 miles on the odometer. Car came in with a P0135 code (Bank 1 Sensor 1 Heater Malfunction). Mr. H tried a new upstream oxygen sensor and the code came back immediately. Time to do a little testing. Since this car only has one bank the chances of the wrong oxygen sensor being changed is cutdown. You don't know how many times I have seen this, especially Nissan vehicles. Sure enough the correct oxygen sensor was changed. First things first. Lets make sure we have battery voltage going to our problem child key on engine off. So I backprobe the heater power wire right at the upstream o2 sensor.
Not exactly battery voltage. Now we know why we were setting the heater code. No battery voltage means no heater operation. Time to break out the wiring diagram.
Now we have some directions. Looks like this circuit has a fuse labeled HO2S that feeds both the upstream and downstream heater circuits for the oxygen sensors. Ok, lets check the fuse. this fuse is located in the underhood fusebox. Checking the fuse with a DVOM tells me the fuse is getting battery voltage and the fuse is good. Now what? Lets divide and conquer. We have a connector and a splice joint between the fusebox and the o2 sensors. I always choose connectors over splice joints whenever possible. Reason being connectors are usually easier to find than splice joints. But first look at the fusebox picture.
There is a hole drilled in the side of the fusebox with a snipped butt connector end attached to a wire. When I see this I always get worried. It usually means someone has been trying to "reengineer" the factory wiring instead of repairing it correctly. My catch phrase for this usually goes something like this-"A 100 years of automotive engineering gone in a second." Now, the integrity of the factory wiring is suspect. I follow this rogue wire around the left front strut tower along the firewall to its destination. Of course this wire was not attached to anything. It was defying the laws of gravity in spots.
And there is our destination of the orphan wire. "Expertly" scotch locked to a wire going to the PCM (Powertrain Control Module). Oh boy. Is this our problem? Is this another problem? Was this the remanants of some long ago disabled anti theft device? Well lets get back to testing. As I was saying I always like connectors over splice joints. So I go after C100-located right behind the left front strut tower.
I locate the heater power feed wire at C100.
As you can see I have the same voltage at the o2 sensor at the power side of C100. The power side being the one that is on the power feed side from the fusebox. So now I know my voltage drop is from the underhood fusebox to C100. Remember, divide and conquer. Do I have a rotted wire under the fusebox? It is located right next to the battery. I can't get the rogue wire out of my head. I know it is linked. Hmmm. I start wiggling the HO2S fuse and.....
My meter still stuck in C100 starts to read battery voltage and every voltage in between. Bingo! I pull the fuse out and my first clue is that it pulls out rather easy. I tweak the fuse a bit and reinstall. Same result. I then inspect the fuse cavity.
It doesn't look like much. I removed the fuse next to it for comparison. The terminal is spread open causing contact issues. There is our voltage drop. Now I know where that rogue wire was going at one time. That extra added bulk at the terminal opened it up. Now that there is just a fuse in there (as intended) there is a poor connection. A couple of tries and I was able to close up the terminal sufficiently. A quick run of oxygen sensor amperage on the scanner and a test drive reviewing mode 6 data confirmed the repair.