http://waterheatertimer.org/Troubleshoot-Rheem-Tankless-water-heater.html
Tankless Troubleshooting Manual
Explanation: Hot Water Outlet Thermistor has malfunctioned. At a minimum check the
following if you get Error Codes 31, 32, 33, & 34:
1. Check for scale build up on the inlet filter screen. This scale build up could also happen
inside the machine. Scale build up on all thermistors will cause them to be inaccurate.
2. IF maintenance mode shows 32 degrees as the temperature, then the thermistor is not
making connection. (Possibly unplugged, possibly broken wire).
3. IF maintenance mode shows 199 degrees the thermistor has a direct short.
Display maintenance information line [5y] Water Inlet Thermistor temperature.
Does the
maintenance
information
indicate the hot
outlet temp?
Is the resistance
to the hot outlet
thermistor OK?
No
Replace Hot Outlet
Thermistor
Yes
Yes
The water heater appears to be
normal. Attempt to operate again.
Replace Printer Circuit Board
56