I have a 2016 Pilot and I was getting the start system problem message on my dash. I found that there was a service bulletin related to the start button and that the warranty was extended to 125000 miles on the button. My Pilot has 165000 miles so I bought an OEM button and replaced it myself after watching some youtube videos. It was very straight forward plug and play process. The only thing I did different was disconnect the battery positive terminal because I thought it would be good measure not to have power to the button while doing the swap to prevent inadvertent turning it on. After the replacement when I push the button it and the dash and button lights up white like when you open the door and get in. But no start. Also no accessory mode if I push the button without pushing the brake. The remote still opens and locks the doors, it will lower the windows but will not do a remote start.. If I push the button without the keyfob in the car it will tell me that, so it does recognize the fob is there. I put the old button back in as it still worked the day before, just giving the error. but now it is doing the same as the new button. The battery is new, but put a jumper on it and tried but still the same result. I have switched buttons back and forth as well as removing and reinstalling the battery terminal to make sure all the connections are good. Not sure where to go next, maybe ECU reset? Some type of relay? (although would not think starter relay since it also does not go into accessory mode)

 

UPDATE - Solved it by resetting the ECU.