I have a Rheem rtg 74dvp but its not heating and the display show me 1 76 and 2 76, and come all the time to 76 F and another times its blinking the temperature, but not heat

Two things- Clean the sensor rods or replace(cheap and easy with YouTube instruction video) and at the same time replace the main program chip (also a easy plug in chip). The whole repair takes two hours for a homeowner and costs about 50 bucks for a chip. These are easy and generally the flame sensor rod is the culprit. The code may also be the limit switch but try EZ and cheap first. See You Tube..
https://www.youtube.com/watch?v=npxcVo7RK2c

You received recommended answer posted ago.
Probably same answer today:
Don't install tankless, especially in location where there is no service technician available unless you expect to heat water by setting tankless out in sun.
http://waterheatertimer.org/Troubleshoot-Rheem-Tankless-water-heater.html

Repair any brand tankless, any error code or problem, same day, yourself for $385:
http://waterheatertimer.org/How-to-install-gas-water-heater.html

Add comment and give link to service manual that you located.

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

Copy following link for general troubleshoot and error codes:
http://waterheatertimer.org/Troubleshoot-Rheem-Tankless-water-heater.html

Error code 36 does not show in any resource, including 2008 service manual, or 2012 product manual.
Check that remote control display is illustrating the correct number.
For example there is error code 35.

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

Error code 36 does not show in Rheem tankless manuals.

Copy following link for general troubleshoot and error codes, and Rheem contact information:
http://waterheatertimer.org/Troubleshoot-Rheem-Tankless-water-heater.html

Error code 36 does not show in any resource, including 2008 service manual, or 2012 product manual.
Check that remote control display is illustrating the correct number.
For example there is error code 35.

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

Usually multiple error codes means PC board failure, but could also be overdue maintenance or other part failure, or simply a combination of events.
before calling service technician, open following link for Rheem troubleshooting resources:
http://waterheatertimer.org/Troubleshoot-Rheem-Tankless-water-heater.html

contact rheem you probably have a circuit board problemb

overheat stat or fan, on fan check for small pinhole in rubber tube best of luck

could you have gotten a natural gas tank by mistake? it would have larger orfices and allow more gas than you need propane has more B.T.U.s than nat gas and requires a smaller orfice
chack with your gas co.good luck.

Yes you'll still need a flow stop unless it is already install in the outlet pipe going out out the heater..

I’m happy to assist further over the phone at https://www.6ya.com/expert/joe_a3369546f8067589