Solved! - Bad downstream O2 Sensor blowing Fuel Pump fuse - Saab 9-5 Bulletin Board - Saabnet.com
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Solved! - Bad downstream O2 Sensor blowing Fuel Pump fuse
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Posted by Mark in Marine [Email] (#1837) [Profile/Gallery] (more from Mark in Marine) on Sun, 12 Jul 2020 15:52:36 Share Post by Email
In Reply to: Re: Hi Bernie - update 7/12, Bernie [Profile/Gallery] , Sun, 12 Jul 2020 13:47:39
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Thanks Bernie (and Bob and DMZ) -

It is a bad rear O2 Sensor (NOT the harness, thankfully). Technically it was the sensor lead, which wore through insulation against the exhaust & transmission case. Somehow a zip tie that used to be there wasn't any more . . . check those when doing other work.

First step in diagnosing, I unplugged the FP only (not the sensor), fuse #15 blew with ignition ON. I unplugged the entire FP harness (the maroon GM sliding lock 9-pin connector), and fuse #15 blew with ignition ON. Then I measured resistance for all nine pins of FP harness to ground. There are only two of the nine that are fully "open", and I was worried. I measured the #15 fuse lower connector resistance to to ground, and that was about 20 Ohms (at least it was definitely NOT open circuit).

Then I removed both O2 Sensor connectors and measured resistance to engine ground for all four pins of each. Both circuits the same - two open, two in the k-Ohm range. Then I measured the pins of the sensors versus ground, and there I found a difference. All four pins for O2 sensor 1 were open circuit with respect to engine ground, but for sensor 2, two pins (heater circuit) were 10 Ohms or less vs ground. I left sensor 2 unplugged and measured the lower connection for fuse #15 versus engine ground, and NOW that pin was at open circuit vs ground. I reinstalled the brake booster vacuum line and left only O2 sensor 1 plugged in. I re-connected the FP harness, and the engine started right up. I cycled the switch several times and did not blow fuse #15.

I already have two new O2 sensors coming (Bosch OE).

I'm REALLY grateful for the help, guys !

Mark in Marine

->Posting last edited on Tue, 14 Jul 2020 12:17:15.
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