986 Forum - The Community for Porsche Boxster & Cayman Owners - View Single Post - Please confirm my diagnosis of one bad O2 sensor

Timbert · 01-09-2016, 04:36 PM

Solved. Thanks to everyone for the help.

Finally got a second set of legitimate Bosch 13806 sensors from Rockauto and installed them after the cats. All four that I replaced (no-name sensors) were either shorted on the heater circuit or at more than 12 ohms (the Bosch ones were 2.5 to 2.8 out of the box and my Durametric report on the resistance with the four Bosch ones installed is right at spec). Plus, at least one had a shorted signal lead that threw lots of errors and Durametric showed a negative voltage rather than the proper swings of positive current. All four of the bad ones would generate voltage with the propane torch, but that's not good enough to keep the CEL off.

I now have proper voltage swings, and the idle fuel trims are .08 and .12 or so, so I may have a very small vacuum leak but not enough to cause a CEL.

So case closed, I hope, on the major issue of rough idle and bad sensors causing the CEL. Note this was a car I purchased recently as a known basket case, so normal troubleshooting procedures had to be modified. I didn't know what had been done to the car, but I learned right away the sensors and the MAF were suspect, and all non-Bosch, but I also had a major vacuum leak at the j-pipe that didn't show up with the smoke test. That's because I used the j-pipe connection to attach my ghetto smoke machine. Duh.

I also learned an easy-to-overlook fact that the heaters are on the same circuit. So an open circuit on one sensor, either front or rear, will also throw the code for its mate on the other side of the car. Like Christmas tree lights wired in series: when one bulb goes out they all turn off.

01-09-2016, 04:36 PM	#14
Timbert Registered User Join Date: Sep 2015 Location: Phoenix Posts: 15	Solved. Thanks to everyone for the help. Finally got a second set of legitimate Bosch 13806 sensors from Rockauto and installed them after the cats. All four that I replaced (no-name sensors) were either shorted on the heater circuit or at more than 12 ohms (the Bosch ones were 2.5 to 2.8 out of the box and my Durametric report on the resistance with the four Bosch ones installed is right at spec). Plus, at least one had a shorted signal lead that threw lots of errors and Durametric showed a negative voltage rather than the proper swings of positive current. All four of the bad ones would generate voltage with the propane torch, but that's not good enough to keep the CEL off. I now have proper voltage swings, and the idle fuel trims are .08 and .12 or so, so I may have a very small vacuum leak but not enough to cause a CEL. So case closed, I hope, on the major issue of rough idle and bad sensors causing the CEL. Note this was a car I purchased recently as a known basket case, so normal troubleshooting procedures had to be modified. I didn't know what had been done to the car, but I learned right away the sensors and the MAF were suspect, and all non-Bosch, but I also had a major vacuum leak at the j-pipe that didn't show up with the smoke test. That's because I used the j-pipe connection to attach my ghetto smoke machine. Duh. I also learned an easy-to-overlook fact that the heaters are on the same circuit. So an open circuit on one sensor, either front or rear, will also throw the code for its mate on the other side of the car. Like Christmas tree lights wired in series: when one bulb goes out they all turn off.