I have a 2013 Nissan Sentra SV. I just got my car back two weeks ago after having my CVT replaced at 86K. Today my check engine light came on while heading to work.

After dealing with Nissan USA (who covered CVT @ 70%) and the dealership (who had my car for four weeks and "couldn't replicate the issue" for the powertrain dept to approve the replacement) for 2.5 months before my CVT was finally replaced, I was not at all excited about returning to the dealership.

I was told that my shift lock assembly was the issue. My shift lock stopped working about three years ago, and rather than pay $700+ to replace it, I've been using a Starbucks coffee stirrer to unlock the gear shifter. I read the post here about using the zip tie to permanently keep the "lock" from engaging, but I haven't ever bothered to take the console apart since the coffee stirrer works fine. The dealership says the check engine light came on because it thinks the gear shifter is shifting out of gear during driving. They want $420 for the part and $280 in labor.

So I have a few questions.
1. Is this legit and is it an immediate concern?
2. Can I just buy the assembly and replace it myself? (already found the part cheaper online)
3. Any ideas why it's been fine for three years and the check engine light is just now an issue?

*I am female and feel like sometimes I'm told things are more serious than they really are. I was also told today it would cost $300 to replace my door handle that "was about to fall off." It's not. The silver outer piece came loose when someone tried to break into my car, but the underlying door handle works fine. I just have to push the silver piece back into place on one end from time to time.
I have replaced my own brake pads and rotors, a rear wheel bearing, spark plugs, etc. If there's a YouTube video or someone willing to guide me as I do the work, I try to do as much as I can myself to save money. Except oil changes... I happily hand over my money for that!

Sorry for the long post. Appreciate any advice you all can offer!

Gotta see what "code" or "codes" may be stored in the car's computer,or ECM. Outside of the Dealerships , the device known as "scan-tool" ,"code reader" that plug into an under dash socket called "ALDL" connector which accesses the ECM's On-Board-Diagnostics aka" OBD II " . a trouble code P- ##### , showing will be pointing to a specific part (s) or "area" of the car, the code number has to be translated verbally though - better code readers do this automatically , and many auto part stores will hookup and show you the codes/ translation , they are instructed usually to NOT CLEAR the CODES.
Digging deeper is what in depth Factory manuals , youtube, Alldata etc. has is the "parameters" and FULL conditions for triggering the specific code , which is as valuable - maybe more valuable than just codes themselves , and many are not experienced in this deeper aspect of diagnostic work and simply replace the part named in the code . Then they realize, just because ECM points to X it maybe Y and Z contributing in full or part.
Bottom line Dealerships "error" maybe the cause of the SES lighting up and your previous situation IS in fact INNOCUOUS - as before and not detrimental to "operation" ??

Go to parts store and get them to read the OBD for codes for free if not in California and WRITE THEM DOWN. Then go online and research what they mean.

The brake/shift interlock should have nothing to do with the code, likely they are smoking you and you can defeat that solenoid on the shifter that does that to make it like old school cars that never did it. The safety feature is simply for cars owned by families with kids to prevent them from putting car in gear. I have that feature disabled on mine to never work again.