The Nissan Path

hey friends. recently my brother drove car for 45min on the highway, SES came on and when he came to a stop, the transmission went into safe mode. pulled over, car wouldn't crank and faintly smelled like burning transmission fluid. towed home and code was u1000. the code was cleared, i cleaned all grounds in the engine bay, then i drove it in LA traffic for 45+ min in hot weather and cold weather and had no problem for 3 days. then, i was on a short 15min drive at night on a cold day and i noticed the transmission was sluggish and the engine was revving to 3500rpm to maintain 65mph, when normally it stays around 1800rpm on that particular stretch of road. got to the highway exit, tranny slightly kicks while coming to a stop. Sure enough, SES comes on again and i knew if i stopped, the tranny would go into safe mode. got to a stop sign, tranny goes into safe mode and i pulled over. had OBD2 reader already plugged in, cleared code but the car would only crank and stop cranking after about 2 seconds. kept throwing u1000 at me and kept attempting to clear it. with car in the "on" position, the gear indicator would not show what gear the car is in. i let it cool for about 15min, then it started, SES gone, and was able to make it home 2 more blocks to park in my driveway. again, i smelled tranny fluid.

i understand u1000 is a CAN error, as i've done much much research while at work for these past 2 days but have not found a solution. what i'm trying to figure out is what it ISN'T before i start taking the dash apart to check the other ground connections. does this only have to do with anything related to the CAN system, or can a malfunctioning ECU relay or crank sensor also cause this problem? Can a failing BCM cause a CAM error that affects the tranny? Does it sound like some electrical component is heating up and shorting? any help is appreciated.

Car info:
2005 pathfinder SE with 174,000 miles. IPDM was replaced before we found out it was the ECU relay, then had the stalling problem again and the ECU relay was replaced. Tranny refurbished at 88k miles due to coolant leak by dealer and have new radiator. replaced tranny fluid at 160k miles, radiator flush at 150k miles. car was driving excellent prior to this, tranny was always shifting smooth and could not even feel it shift, along with plenty of power from the engine while getting the same 15/21mpgs as stated by nissan. i just checked the radiator and the coolant is still neon green. checked tranny fluid but it is light brown and smells burned.

CAN system codes can be hard to diagnose, especially if you don't have a diagnostic scan tool (and an understanding of what they are and experience in diagnosing them). Essentially, CAN communication works by multiple control units communicating with each other via a pair of wires that are twisted around each other. Diagnosing involves checking the codes of all of the control units in the CAN system and using the chart in the factory service manual to isolate the possible circuits and/or control units involved. It's both a good and a bad thing; by doing this, it eliminates a lot of wiring and replaces it with the two and, if you know what you are doing, simplifies the diagnosing. But, it involves a lot of different system and can be a bit overwhelming to diagnose for someone not equipped to do so.
Of course, we have the U1000/U1001 TSB that would include your Pathfinder, among a number of other mid-2000's Nissan models and involves cleaning a bunch of ground points in the engine compartment and behind the dashboard. If that doesn't fix the problem, it helps that you do have the consistent transmission problem that kind of leans you toward an issue with the TCM to ECM communication. You saying the fluid is brown and smells burnt is concerning; if the fluid is burnt, you may have a bad transmission that is causing the issue (even though it's not that old), so it might be worth having a reputable transmission shop take a look at it. Outside of that, it's follow the diagnostic procedure in the FSM to identify the problem. From my experience, problems with the twisted wires (being open or shorted) are somewhat rare and control unit failures are a lot more common. That said, you don't know until you do the diagnostic work.

awesome - thanks for the reply. i guess it's time to start tearing apart the dash. i'm hoping something in the CAN line failed which caused the tranny to overheat versus the tranny overheating causing the TCM to fail. man i love this car, but owning it since '05 has been made it hard to want to keep around at my mileage.

the U1000 code is almost impossible to diagnose without a very good scan tool. shoot me a PM if you need further help