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This ended up being a multi-faceted issue for me.

The oil viscosity sensor was indeed blown and causing a short. This was replaced - and the BCM was replaced as it showed up bad on a diagnostic run by my studio. They also discovered a leak in the rubber boot that covers the wires that go into the hatch that also caused them discover a frayed wire in that bundle. They sealed up everything, mended the wire and put everything back and so far so good. Another symptom that showed up was my dome lights going out and not working.
So we can either change the sensor or cut the wire? Why change the sensor if ( from what i understand) has been bypassed by software and hence no longer used?

thanks for the clarification!
 

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Thanks for the reply! So the viscocity sensor and dome lights are on the same circuit? Have to find a wiring diagram!!
 

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Yeah, that's my understanding or at least the short in the oil sensor causes other problems downstream. Either way, it's a good place to start looking. Remember, these are Italian cars - don't look for logic! :D
 

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Update for everyone...i ended up cutting the red wire that goes to the oil sensor...and....my lights work!!!!! Crazy but TRUE!! Thank you all for the help.

Dave
 

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I can confirm that cutting the red wire solves this issue and doesn't seem to harm anything. My wife's 500 Sport kept blowing fuses for the dome light immediately, so I took the advice I found in this thread (plus in a related thread elsewhere talking about why the oil viscosity sensor is no longer used) and cut the red wire.

I was just going to pull the sensor and disconnect it, but I couldn't get the thing free, even when prying with a screwdriver while pushing on the little tab. When I opened up the wire protection sleeve, I found that the red and purple-ish wires had somewhat melted together and created a short. At first, separating the wires allowed me to put in a 5-amp fuse without blowing it, but as soon as I buttoned everything up and lowered the car, I noticed that the dome light had stopped working. (Hopefully this saves someone else a headache later!) I went ahead and cut the red wire, wrapped both ends in electrical tape, and put it all back in the plastic sleeve. We drove the car around this evening with no engine problems, and the dome light continued to work.

I'm attaching a picture of the snipped wire as proof for anyone worried about cutting it, and for future reference in case anyone needs another view. Hope this helps someone like you guys have helped me!
 

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I just don't think that I could do it.

TEdolph
Right? It's definitely not logical, but it worked in our case. Best I can figure is Fiat put that sensor on the dome light circuit temporarily during the engineering process, thinking it wouldn't have much load, but they didn't account for what happens when that sensor goes bad. Then, when they discovered the sensor isn't needed, they just removed it from the documentation. Having worked on my 100-year-old house a ton and discovered lots of "interesting" choices in the wiring, I no longer expect electrical stuff to make sense...

And I should mention I cut the wire at a spot that would still let me reconnect it with a butt connector if needed. Can't be too careful!
 

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To test the sensor what do I test for OC or CC?

OC I assume?

Some resitance value?

TEdolph
I'm not sure, sorry! I couldn't even get the wire clip off of mine. I think it melted together due to the short. Can you get yours loose? If so, I'd start by just testing for the presence of voltage with the key off. In ours, the fuse was blowing even when the car was completely off. Not sure why there would even be voltage to an engine sensor with the power off, but whatever!
 

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I have come across this problem before and it is only on the early build 2012 500 with the oil viscosity sensor in the oil-pan. The sensor is shorted but is not used by the p.c.m. anymore so no codes are set. Raise the vehicle, remove lower shield & cut the red/orange wire on the 3-wire connector of the oil-visc sensor on the passenger front corner of the oil-pan. This will remove the short from the light circuit, not cause any problems and fix the blown fuse problem.
This posting has been very useful to me. THANKS SO MUCH! It is now 2021 and I am having the same problem with my early model year 2012 Fiat 500 Sport (70k miles). Even finding the interior fuse panel was a chore because it is tucked under the dashboard in a manner that renders it invisible unless you get way down under the steering wheel and look up (none of the manuals or YouTube videos show fuse boxes in this location). Will get under the car and cut the wire and hope that the short will go away. I also had a parasitic current drain problem, which I hope will disappear when I fix this problem. What I do not understand was why Fiat did not document this part which was later discontinued, and also why they did not correct the issue way back in 2014 when it was coming up for other owners and my car was still under warranty. I love my vehicle -- it is an excellent city car for Boston, where pa king the ca is haaa d, but it has had an inordinate number of issues. I think the moral is to never buy a car that is early in the production run, before all the bugs have been worked out.
 
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