Code 1635 is telling you is the 5 volt reference signal is low. Either a sensor is pulling it down or wiring issue. Could also be corrosion or a ground.
You would need to monitor the 5volt reference signal with a scan tool and unplug the sensors one at a time to see when the 5 volts returns.
For the p1236 Check that all your sensors are plugged in and seated from removing the intake.