Hi this is my first post, I read the forum etiquette tips but please let me know if there is anything I should do differently when posting.

I'm looking at a 2016 Subaru Crosstrek equipped with a CVT. The backup camera wasn't working and I recently attained my A6 ASE certification and wanted to try my hand at electrical diag. Another technician suggested I just throw a camera at it because in his experience its usually component failure. I had a feeling this wouldn't fix the problem and it didn't. I dug deeper into the service information and found that the reverse lights send a reference signal to the radio/touchscreen to display the backup camera when the gear selector is in reverse. I had an assistant tell me whether or not the backup lights work and they were not working. I checked the fuse for the back up lights and it was blown (this fuse also powers the inhibitor switch, as well as the trailer connector (when equipped) and the auto head light leveling system (when equipped)). This particular vehicle is not equipped with a towing package or an auto headlight leveling system. I thought it was possible the fuse was blown from a random voltage spike created by shifting in or out of reverse too fast (similar to accidently arcing the plug for a cellphone charger with the cars cigarette lighter and blowing the cigarette fuse) but again I had my doubts that just replacing the fuse would fix the problem. The fuse did not immediately short out, in fact the reverse lights and backup camera worked fine for a day, and then the fuse blew again the following day. Performed some diagnostics in STIS (Subaru's factory service information) on the inhibitor switch (essentially the neutral safety switch but on a CVT) and had continuity at specific harness terminals in specific gear ranges. Next I performed a continuity test involving the neutral range, there is a "gray area" so to speak, between neutral and reverse and between neutral and drive, when the gear selector is fully engaged in neutral there is supposed to be continuity, when the gear selector is in one of the "gray areas", meaning rock the gear selector towards reverse or drive 1.5 degrees but don't engage, there is supposed to be no continuity or infinite resistance. I found I had continuity in these "gray areas" and checked the adjustment of the inhibitor switch. The inhibitor switch was adjusted properly as per STIS, and the next step in the trouble tree was to replace the inhibitor switch. After doing this I thought maybe the fuse would stop intermittently blowing and that the faulty inhibitor switch had an internal short to ground and was the root cause. Unfortunately this did not solve the problem, there is still an intermittent short to ground somewhere in the circuit. If it were a direct short to ground and the fuse blew all the time, I know I could wire a low wattage headlight bulb in place of the fuse and unplug connectors one at a time until the bulb went out, which would help me narrow down the part of the circuit with the short to ground. Being that this is happening intermittently, I cant really perform this test.

I've only witnessed the fuse blow in reverse, that's not to say its not possible for this fuse to blow in another gear range, I just haven't personally seen it happen. When the short is happening, it will do it for 1-3 hours, then it will magically refuse to blow for another 6-8 hours (talk about frustrating)

I'm posting this in the discussion forum with some background on the problem rather than TechHelp because I honestly don't expect a one sentence answer with an immediate fix. I'm more or less curious whether anyone else has had this exact problem and also curious what the general strategy is for finding intermittent short to grounds. Based on what I read in some online articles, there is no diagnostic strategy for finding intermittent problems, the only solutions are to throw parts at the car until the problem goes away, or tell the customer to just "live with it" until the circuit fails completely and becomes more diagnosable. I also don't feel confident enough to offer help in TechHelp before posting my own help question (I've only been turning a wrench for 7 years, I consider myself a professional but I'm on the low end of the experience scale). Again, I'm looking more for a general discussion on tracing intermittent shorts rather than a magic bullet answer.

Nathaniel Cole
Technician
Carbone Subaru of Troy
Troy, New York, USA