2023 SP; new 2WDW flash, bike won't start. Suggestions?

I'm also not the best mechanic, but I did notice that there's a check engine light on. I would recommend to get a cheap code reader and maybe this can give us some more hints as to what might be the issue.
www.amazon.com/dp/B09GX4PVQ1?psc=1&ref=ppx_yo2ov_dt_b_product_details <- this adapter allows you to use normal OBD2 readers. I used this and it worked with my OBD2 reader.
I don't have a specific recommendation for a code reader, but these bluetooth ones are pretty good - www.amazon.com/Veepeak-OBDCheck-Bluetooth-Diagnostic-Supports/dp/B073XKQQQW/ref=sr_1_5?crid=2C1C9YEXTBZ0A&keywords=obd2+scanner+bluetooth&qid=1700345453&sprefix=obd2+scanner+bluetooth%2Caps%2C81&sr=8-5
I ended up using this app - apps.apple.com/us/app/car-scanner-elm-obd2/id1259933623 - with my friend's OBD2 scanner (just gave me the same info the cheap one I have did)
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You also mentioned that you disconnected/reconnected the battery. Is it possible that you swapped the negative/positive? If that happened, a fuse is surely to be blown. If possible, check your fuses.

Other than those extremely basic troubleshooting steps, I'm not sure. Make sure the battery terminals are tight and make sure the connections to the ECU are tight.

The check engine light will go off after 2 running cycles. It needs to get up to full temperature too.

Yeah, as per 2WDW's page for the flash:


I only disconnected the negative lead and didn't remove the battery, so there's no way the battery could be connected backwards. 

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Since my original post, at 2WDW's direction, I've removed and reinstalled the ECU just to be sure it's fully power cycled and connected properly.  Same results.  No error messages shown, just won't fire when turning over.  There ARE "problem with ECU" fault indicators that exist on the dash according to the manual ("Err" should show on the top bar, and a wrench icon, if there's a problem communicating with the ECU), but none of them are illuminated.  

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Regarding OBD2 readers, I *strongly* recommend Scantool LX OBDLink readers.  I've used a lot of the other ones over the years, but the Scantool models are the only ones that actually sleep when the bike is off (so you can leave them plugged in) and either their OBDLink software or Torque OBD.  Sadly, my prior Yamahas all had 4pin connectors, whereas the MT10 has a 6pin, so my adapter won't work and I'm still waiting on a correct one in the mail.  So, I can't check it, but as per the quote above, a check engine light is expected.

They'll get it sorted. Nate over there is an awesome dude. He's pretty active on the FB MT groups as well, I think he started one of them (he's a mod on it at least).

Final update on the ECU saga:

Got it back after a lovely week of Canada Customs being... Canada Customs. Reinstalled it, the bike fired up instantly and runs *extremely* well. The difference is astounding - I noted above I've had one of their flashes before on my MT07 and loved it, but this is nothing like that. The ETV changes as well as the power increase across the board, oh good lord, it's outstanding. No longer does it have that kind of awful kick when it finally opens the throttle around 6-7k; it just does what you tell it to, when you tell it to. Way more exciting <7000rpm, and way more linear power across the whole rev range. The smoother torque curve is abundantly evident.

And, because you can actually give it throttle at lower RPM now, it makes that wonderful noise at lower RPM too once you've got the engine under load.

100% worth the extra $100 in shipping and downtime. No regrets whatsoever.