2008 pathfinder crank no start

[bearden08]

2008 Pathfinder SE 178K. Runs great. Never had any issues. Recently stopped to fuel up and then it wouldn't start. Had it towed back to the house. It will turn over for a about 3 seconds then stops. My computer reader will not connect with the PCM. I have tried multiple readers but none will connect. Made me suspect a bad PCM, so I swapped the PCM with a known good one (from a friends car). Computer reader still doesn't connect. Also swapped the fuse box relay block (near the computer on the firewall), still doesn't connect with computer reader. The engine light has been on for about a year with a code for a bad fuel temperature sensor which is in the fuel tank on the fuel pump. Haven't changed the temp sensor because I dont really want to drop the fuel tank. Any help is much appreciated.

After doing some research, I saw were this issue could be associated with the security system.

[VStar650CL]

The fact that it cranks and stops kind of leaves out a security issue. If it's an I-Key car then it shouldn't crank at all with a security problem, if it's a hard key then it should crank normally but not start (the ECM kills spark). In either case you should get a solid red security light if that's the issue. You actually may have created a security problem by swapping ECM's. The swapped one won't recognize the key at all, and on some models that makes the original ECM refuse the key once it's reinstalled.

That all said, your first step should be to make sure you have voltage and ground at the OBD port (DLC). Pins 16~4 and 16~5 should light a test lamp at all times or fuse #19 is probably blown. Pins 8~4 and 8~5 should light a test lamp with key on or fuse #12 is probably blown. You can find the wiring diagram for the DLC here on page GI-61:
https://www.nicoclub.com/service-manual ... 8%2Fgi.pdf

If that all checks out, measure the voltages at pins 6 and 14 with the key on. I didn't look up which one is CANH and which is CANL, but one (CANH) should read slightly above 2.5V and the other (CANL) should read slightly below 2.5V. If you read anything else then it's likely some device is crashing the CANbus. The transmission controllers (TCM's) on R51's are notorious for killing the whole bus when they fail, so you might try unplugging the transmission umbilical and see if the scanner connects all of a sudden. If so, the TCM is crapped.

[bearden08]

I guess I miss worded my original. When i try to start it, it just spins over for a few seconds and doesn't actually start. Doesn't even try to start. Same condition when swapping the ECM.

[VStar650CL]

It isn't normal for it to spin "for a few seconds" under any circumstances. If it's an I-Key with a security problem it shouldn't spin at all, if it's an I-Key with a no-start engine then it should spin until the BCM times out (about 10 seconds on most models), and if it's a steel smartkey then it should spin for as long as you hold the key on Start. That makes it likely your no-connect scanner is a symptom of whatever is making it misbehave. Solve that and chances are you'll also solve your no-start.