What should I fix?

Hi, my name is Devin. I am an auto repair expert with the Car Genius team here at RepairPal. So sorry to hear you're having trouble with your Yukon. The codes you see, P0191 and P2635, indicate issues with the fuel system. P0191 points to a problem with the fuel rail pressure sensor, which could mean it's not reading the pressure correctly or there's an issue with the fuel delivery. P2635 is related to the fuel pump, suggesting that the pump might not be delivering fuel properly. Given that your Yukon stopped working and won't start now, the fuel pump could be failing, preventing the engine from getting the fuel it needs to run. The electrical issues you're describing, like the headlights staying on and the ignition not responding, could point to a problem with the battery or a related electrical issue, possibly a short or a stuck relay. To get your Yukon running again, you should start by inspecting the fuel pump and the electrical system. A more thorough diagnosis by a professional might be required to pinpoint the exact issue, especially with the electrical components. If you need more advice, have any other questions about your vehicle, or would like to get connected to a RepairPal Certified shop near you, we have you covered. Give us a call at 877-913-1554, and we will happily help you out!