Ruud Error Code 15: Inlet Temp Sensor Fault

Severity
Warning
Type
digital display
Estimated repair cost: $100 — $250 (parts + labor)
Warning: Your Ruud tankless water heater has detected a problem with its inlet water temperature sensor and has shut down to prevent potential issues. You'll need to troubleshoot or call a technician to get hot water again.

How to Fix Ruud Code 15

  1. Step 1: Check the display periodically to see if the error clears itself. Sometimes transient electrical noise can cause false errors.
  2. Step 2: If you are comfortable, check the wiring connections to the water heater. Make sure the power is off. Look for any loose or corroded connections. Do not disassemble anything, just visually inspect.
  3. Step 3: If the error persists, contact a qualified HVAC technician to diagnose and repair the issue.

How to Reset Ruud Code 15

  1. Turn off the furnace power switch, usually located on or near the unit.
  2. Turn off the circuit breaker for the furnace. Wait 30 seconds.
  3. Restore power at the breaker and flip the furnace switch back on.
  4. Set the thermostat to call for heat. If the error code returns after 2-3 ignition attempts, the fault requires repair.

Resetting clears the code from the display but does not fix the underlying fault. If the code returns after reset, proceed to the diagnostic steps above.

What Does Ruud Error Code 15 Mean?

The '15' error code on your Ruud tankless water heater indicates a fault with the inlet water temperature sensor. This sensor is responsible for monitoring the temperature of the water entering the unit. The control board uses this information to regulate the burner and ensure proper operation. If the sensor is faulty, the water heater may not be able to accurately control the water temperature, potentially leading to overheating or inefficient operation. Because the unit can't verify safe operation, it locks out until the issue is resolved. This protects the unit from damage and, more importantly, protects you from scalding water.

Common Causes

  • Faulty inlet water temperature sensor
  • Loose or corroded wiring connections to the sensor or control board
  • Damaged wiring harness leading to the sensor
  • Control board failure
  • Scale buildup on the sensor, insulating it from the water

Diagnostic Steps

If... Check Expected
Unit displays error code 15 Measure the resistance of the inlet water temperature sensor at the control board connector. Resistance should match the temperature/resistance chart in the service manual. Typically around 10k ohms at room temperature.
Sensor resistance is out of range Inspect the wiring harness and connectors for damage or corrosion. Wiring and connectors are in good condition.
Wiring is good, sensor resistance is still bad Replace the inlet water temperature sensor. Error code clears after replacement.
Error persists after sensor replacement Check the voltage at the control board connector for the sensor. 5 VDC

Applicable Ruud Models

Replacement Parts

Part OEM Number Est. Cost Where to Buy
Inlet Water Temperature Sensor REU6109 — Find Part
Control Board — — —

Related Codes on Same Models

Other codes that affect the same Ruud model families as 15.

Code Description Severity
11 No Ignition Critical
12 Flame Failure Critical
13 Improper Combustion Critical
14 Overheat / Thermal Fuse Critical
16 Outlet Temp Sensor Fault Warning
21 Air Supply / Operational Switch Warning
24 Flow Sensor Malfunction Warning
29 Condensate Neutralizer Issue Warning
31 Inlet Water Filter Warning
33 Fan Motor Fault Warning