My heater cuts on and off and give me code of r-13

1) Copy following links for Rheem tankless error codes
http://waterheatertimer.org/Troubleshoot-Rheem-Tankless-water-heater.html
http://waterheatertimer.org/pdf/Tankless-Error-code-13.pdf

2) Maintain water heater as shown in product manual to avoid common error codes.

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Before calling service technician, open following links for Rheem tankless resources:
http://waterheatertimer.org/pdf/Error-code-13.pdf
http://waterheatertimer.org/Troubleshoot-Rheem-Tankless-water-heater.html

Before calling service technician, open following links for troubleshooting resources including Rheem chat and typical manuals:
http://waterheatertimer.org/pdf/Error-code-13.pdf
http://waterheatertimer.org/Troubleshoot-Rheem-Tankless-water-heater.html

All gas water heaters require periodic maintenance.
Tankless recommend yearly maintenance.
Typical maintenance would cover most the causes for error 13, plus causes for most other common error codes.

Before calling service technician, open following link for Rheem troubleshoot resources including error codes, Rheem chat, and possible manual:
http://waterheatertimer.org/Troubleshoot-Rheem-Tankless-water-heater.html

Error codes are considered start-point for troubleshooting.
Error code C4 13 is bad oxygen sensor, inadequate incoming air supply, or bad venting. Also bad PC board, or internal wiring, or temperature set too high, damaged burner parts.
http://waterheatertimer.org/pdf/Error-code-13.pdf
http://waterheatertimer.org/pdf/Bosch_Ventilating_tight_houses.pdf

Tankless require yearly maintenance to combustion chamber and burner parts.
Prescribed maintenance can correct some of the issues in error code 13.

Multiple error codes can mean different things, including bad PC board.
C2 13 might be the oxygen sensor. -0 and no operation indicates bad board, but other things might be notched into same problem like loose connections.
Error codes are starting point for troubleshooting.
Before calling service technician, open following link for Rheem-Paloma troubleshoot resources including Rheem chat.
http://waterheatertimer.org/Troubleshoot-Rheem-Tankless-water-heater.html

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Internet doesn't have clear-cut solution for C7 13 error code.
If concise repair information or service bulletin comes available, I will update this thread.

Open following links for Rheem troubleshooting resources:
http://waterheatertimer.org/Troubleshoot-Rheem-Tankless-water-heater.html

problem is inside burner compartment o2 sensor dirty or block..need to have a qualify technician clean it.

I have a code showing C7 13 the heater runs for about 2 to 3 mins then shuts off. Any suggestions?

4 error message, the 4th (last) error code is #13, not enough oxygen.

Check the location of the heater, if it is fine the ODS sensor (actually is a thermal couple) is bad.

Maximum temp exceeded on ntc1(thermistor),,,either clean out heat exchanger,power flush of my first option replace thermistor...