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Do you have any vapors in the area of the heater flammable or otherwise?
OK, assuming the sensor is ok the reset procedure is as follows:
Turn the power off/unplug the heater then turn the temp dial all the way down. Wait a minute then power it back up. If it doesn't reboot then turn the dial all the way up and all the way down 7 times within 30 seconds. If this doesn't reset it then you either have a bad sensor or gas control. If that is the case then this should be a warranty issue and you'll need to call Rheem in the morning. Call the number that's in your manual. If you do call them don't mention you tried this reset procedure since this knowledge is only supposed to be available to service techs.
Good luck, this is right out of the Rheem service bulletin
I can only communicate with you through this interface. Since this isn't your house how do you know for sure vapors (flammable or otherwise) aren't the issue. If they are indeed the issue then you have to air out the place completely. If you have a multi meter you can check the resistance across the sensor If it doesn't fall between 9k ohms and 45k ohms then it's no good. Sensor is connected to the 2 white leads off the bottom of the gas control.
If you have an ohm meter that would confirm if the sensor is good or not. Without testing the sensor you won't know the cause if it doesn't reset.