Can I still get hot water with Rheem code 31?

Usually not reliably. Without a valid inlet temperature, the unit cannot control its output properly.

Rheem Tankless Code 31 (Inlet Thermistor): What to Do

A tankless heater needs to know how cold the incoming water is to calculate how hard to fire. The inlet thermistor provides that reading. Code 31 means the control board is getting a reading it cannot trust.

The meaning and service steps below come from Rheem's tankless gas troubleshooting manual, which covers both Rheem and Ruud tankless water heaters. Rheem technical support: (800) 432-8373.

What a technician checks

Resistance through the sensor circuit at the control board connector. Rheem's range is about 7,000 to 23,000 ohms.
If that is out of range, the reading at the sensor's own connector near the blower, to tell a bad sensor from a bad harness.
Scale on the thermistor itself. Rheem's chart includes cleaning scale buildup before replacing anything.

Depending on which reading fails, the thermistor, the wiring harness or the control board is replaced.

The scale connection

Scale on sensors is a sign the whole heat exchanger may need attention. If your water is hard and the unit has not been flushed in a year or more, ask for a descale at the same visit. Rheem also has a separate warning code, 1L, that appears when the control board detects lime buildup in the heat exchanger.

How to reset it

Close all hot water taps.
Turn the remote control off, wait about 10 seconds, then turn it back on.
Open a hot tap and watch whether the code returns.

Before you call

Have the model and serial number from the rating label ready, along with the exact code and when it appears: at startup, during use, or only at high demand. Mention any recent work on the gas, water or electrical supply and when the heater was last serviced. This helps the technician bring the right parts and often saves a second visit.

See every code on our Rheem error codes page.

Other Rheem codes

Code	Meaning
00	Continuous combustion for more than one hour
03	Communication failure between units, remote or manifold controller
05	Improper combustion detected by the flame rod (warning)
10	Air supply or exhaust blockage (hard lockout)
11	Ignition failure
12	Flame failure
13	Poor combustion detected by the flame rod (indoor units)
14	Overheat safety tripped
15	Heat exchanger or hot water temperature too high (boiling safety)
16	Outlet water temperature above the set point (over temperature warning)
21	Air intake filter switch fault (earlier models)
24	Remote control buttons held too long, or operational switch fault
29	Condensate not draining (condensing models) or heat exchanger temperature too low
31	Inlet water temperature sensor (thermistor) fault
32	Heat exchanger thermistor fault
33	Outlet (outgoing water) thermistor fault
34	Ambient or combustion air temperature sensor fault
35	Improper thermistor connection
51	Gas valve fault: flame detected after shutdown (current) / gas inlet solenoid valve fault (earlier)
52	Modulating gas valve (PGFR) fault
61	Blower (combustion fan) motor fault
65	Water volume control valve fault
66	Water bypass valve fault
71	Gas inlet solenoid valve fault
72	Flame rod fault: flame sensed before ignition
76	Remote control communication fault
79	Blower (fan) motor current fault
80	Gas cut-off failure: flame detected after the unit turned off
82	Control board program chip fault (current) / gas type data failure (earlier)
90	Vent blockage detected before ignition
92	Neutralizer needs replacing soon (warning, condensing models)
93	Neutralizer must be replaced; unit will not operate (condensing models)
1L	Lime (scale) buildup detected in the heat exchanger
99	Vent blockage detected after shutdown / blocked flue (hard lockout)
P1	Not enough water flow to fire the burner (warning)

Rheem code 31: inlet water sensor fault