Wife ran errand with her car yesterday. Very, very cold out but car started and drove ok after she let it warm up for 5 minutes. She parked, ran her errand and when she came back it wouldn't start. I checked battery and it had 12.7v according to multimeter. Connections weren't corroded but I cleaned anyway. Still no crank, no start. Everything else works. Went back with OBD scan tool and it can't connect. Checked OBD scan tool on my truck and it connects. Checked all fuses and fuseable links and they all appear to be good. Is there a relay or something that'll kill the starter AND the OBD port? Really need some help with this one.

Yes, the ECM relay could cause that, as well as a bad ignition switch if it's a steel-key car. There's also a 20A fuse in the IPDM dedicated to the ECM Relay, usually labeled "EGI". If it's blown then you need to find out why, usually it means a wiring short or a shorted coil in something the ECM is driving like the IVT/EVT solenoids.

You'll find the engine control wiring diagram here starting on EC-114:
https://www.nicoclub.com/service-manual ... 3%2Fec.pdf
You'll find the power supply, ground, and fuse layouts here:
https://www.nicoclub.com/service-manual ... 3%2Fpg.pdf

Thank you for the info!

Is there a way to test that relay? I suppose I could steal the relay out of my Frontier (assuming they're the same).

I'm not sure if it's accessible, there are two unlabeled relays on the IPDM layout and it might be one of them, or it might be internal to the IPDM. One of the Juke guys here might know the answer to that. Assuming fuse 43 is good, if you look on EC-115, the ECM Relay feeds two wires out of the IPDM, pins 35~36. You can locate them in the diagram on PG-33. Put a bulb-type test lamp between those wires and ground with the key on, the lamp should light. If it does, then the relay isn't your problem.

Just went out to see if I could start diagnosing. Car started and ran fine. OBD scanner makes connection fine. No codes though.

No idea what to do now. If we do nothing we run the risk of this happening again when we're much farther from home. But if we take it to Nissan they likely won't find anything because everything is working (I also don't trust our local Nissan dealership).

If your scanner is OBD-II then it most likely only interrogates the engine, but if the ECM went down there will be codes in other systems like the BCM and IPDM to clue you in. So I'd suggest letting the dealer (or a place with a good shop scanner) run codes for the whole car. Aside from that, all I can suggest is do some preparation to test the most likely diagnoses if it reoccurs.