Rheem 11 Error: Lower Thermistor Sensor Resistance Drift
Troubleshoot Rheem water heater code 11 (lower thermistor fault).
Lower temperature sensor failure. The smart control module detects that the resistance of the lower NTC thermistor is out of normal operating range (shorted or open circuit).
Probable Causes
Lower NTC thermistor burned out or open-circuited due to thermal aging.
Moisture or condensation corrosion on sensor pins inside the dry well cavity.
Pinched or severed 2-wire sensor harness between lower tank well and upper control board.
Diagnostic & Testing Steps
- •EcoNet digital screen or smartphone app displays Alert Code 11.
- •Status LED flashes 1 time, pauses, then flashes 1 time.
- •Water heater operates in limited backup mode; recovery time is delayed.
Tools & Equipment Required
- ✓Digital Multimeter with µA DC RangeMeasure flame ionization current (min 1.0 µA DC) and check ignition spark gap (3-4 mm).
- ✓Digital Differential Gas ManometerVerify gas inlet working pressure under full tankless firing demand.
- ✓Phillips #2 Screwdriver & Slotted 4mmOpen front cabinet door and detach burner sight glass / electrode plate.
- ✓Brass Wire BrushClean carbon deposits from flame rod and ground electrode.
- ✓Gas Leak Detection SprayInspect all gas valve manifold joints after diagnostic checks.
Fast Reset Sequence: Universal 5-Minute Power Isolation Reset
Before unmounting panels or beginning diagnostic procedures, attempt this manufacturer reset to clear transient microcomputer lockouts:
- 1.Press the Power or Cancel button to interrupt the active cycle.
- 2.Completely unplug the appliance or trip the dedicated wall circuit breaker.
- 3.Leave disconnected for 5 minutes to discharge internal power supply capacitors and reset microcontrollers.
- 4.Reconnect power and run a test cycle to confirm whether the error is transient or hardware-related.
- 1
Step 1: De-energize 240V Breaker and Access Lower Sensor
[ ] DoneSwitch off double-pole breaker, remove lower access panel screws, and fold back fiberglass insulation.
Verification:Multimeter confirms 0 VAC across lower element screw terminals and ground. - 2
Step 2: Measure Thermistor Resistance with Multimeter
[ ] DoneUnclip 2-pin molex connector from thermistor and measure resistance across pins on 20kΩ range.
Verification:Reading measures 9.5 kΩ to 11.5 kΩ at room temperature. Open circuit (OL) indicates failed thermistor. - 3
Step 3: Install Replacement Thermistor Probe and Test
[ ] DoneSlide new thermistor probe firmly into the dry well spring clip, reconnect wiring plug, and restore power.
Verification:Code 11 clears from EcoNet display; control module accurately registers lower tank water temperature.
Repair Solutions
Turn off 240V breaker, remove lower access cover, and disconnect sensor plug.
Measure ohmic resistance of thermistor with digital multimeter on 20kΩ scale.
Standard healthy sensor reads ~10.0 kΩ at room temperature (77°F / 25°C); replace if reading is 0 Ω or infinite (OL).
Frequently Asked Questions
Do I need to drain the tank to replace the lower thermistor on a Rheem Gladiator?
No. The thermistor sits in an external dry well welded to the tank shell; no water drainage is required.
Can the water heater still provide hot water with Code 11 active?
Yes. The water heater defaults to upper-element-only heating mode, but hot water recovery capacity is reduced by approximately 50%.