Why did Rheem code 35 appear after my heater was serviced?

A thermistor connector may have been left loose or plugged into the wrong spot.

Rheem Tankless Code 35: Thermistor Connection Fault

Code 35 is usually a connection problem, not a failed part. It is common after someone has worked inside the unit.

This page uses two Rheem sources: the current Rheem and Ruud tankless troubleshooting manual, and Rheem's error code list for earlier tankless models. Codes can differ between generations, so the meaning for each is shown where they differ. Rheem technical support: (800) 432-8373.

What it means by model generation

Rheem generation	Meaning
Current models	Improper thermistor connection: one or more of the four thermistors has a poor connection or is in the wrong location
Earlier models	Improper thermistor connections

The four thermistors

Sensor	Its own fault code
Inlet water	31
Heat exchanger	32
Outlet water	33
Ambient	34

What a technician does

Turns off power and removes and reinserts the thermistor connector on the control board.
Confirms each sensor is plugged into its correct location.
Tests the sensors if the code returns.

If it appeared after a repair

Call the technician who did the work; a connector was likely left loose or swapped.

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, and note whether your unit is a condensing (high-efficiency) or non-condensing model. Tell the technician the exact code, when it appears and any recent work on the gas, water, venting or electrical supply. That 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 35: thermistor connection fault