Troubleshooting

Rheem RTEX-24 Error Codes

An independent, quick-reference guide to diagnose heating faults, flashing codes, and plumbing issues safely.

HIGH VOLTAGE ELECTRICAL SAFETY WARNING

The Rheem RTEX-24 operates on **240V electricity** powered by **three double-pole 40-amp circuit breakers**. Always turn off all three circuit breakers completely and use a certified non-contact voltage tester to verify the power is entirely dead before removing the front cover or attempting any internal inspections, diagnostic tests, or heating element replacements.

Quick Reference Index

Error Code / Fault Status / LED Severity Quick Meaning Action
E1 Flashing Red (Display) Danger Inlet thermistor/sensor fault. The unit cannot measure incoming water temperature. View Fix ↓
E2 Flashing Red (Display) Danger Outlet thermistor/sensor fault. The unit cannot monitor hot water outlet temperature. View Fix ↓
E3 Solid Red (Display) Danger Over-temperature protection triggered. Outlet water exceeds 140°F (60°C). View Fix ↓
E4 Flashing Red (Display) Warning Heating element failure or dry fire detected. Elements draw no current. View Fix ↓
E5 Flashing Red (Display) Info Flow sensor failure or water flow is below the minimum activation threshold (0.3 GPM). View Fix ↓
No Display Blank Screen / No LEDs Danger Control board is not receiving 240V power or the main display fuse is blown. View Fix ↓

Detailed Troubleshooting Steps

E1

Inlet Thermistor Fault

Danger

What it means: The control board detects an open or short circuit on the inlet temperature sensor wire plug. Heating is suspended for safety.

Common Causes:
  • Thermistor wire harness plug came loose from the circuit board socket.
  • Corroded thermistor terminals or damaged sensor wiring.
  • Defective inlet thermistor assembly.
How to Rectify:
  1. Turn off all three 40A breakers completely.
  2. Remove the front cover and check the inlet sensor connection on the main board.
  3. Use a multimeter to measure the thermistor resistance (should read ~10k ohms at 77°F/25°C). Replace the sensor if it reads open or shorted.
E2

Outlet Thermistor Fault

Danger

What it means: The control board cannot read outlet water temperature, creating an overheating risk. Power output is stopped.

Common Causes:
  • Loose wiring plug at the control board socket.
  • Scale buildup on the thermistor body inside the pipe fitting.
  • Damaged outlet sensor wiring harness.
How to Rectify:
  1. Turn off all three circuit breakers.
  2. Check the wiring connection on the main circuit board.
  3. Measure outlet thermistor resistance. If resistance is incorrect or read as open/infinity, unscrew the thermistor and replace it.
E3

Over-Temperature Cutoff (ECO)

Danger

What it means: The outlet water temperature exceeded 140°F (60°C). The thermal cutout switch has tripped to prevent scalding.

Common Causes:
  • Low flow rate coupled with high temperature setting.
  • Severe scale buildup on the heating elements insulating the heat transfer.
  • Defective control board relay stuck in the ON position.
How to Rectify:
  1. Wait for the unit to cool down. Turn off power completely.
  2. Press the red manual reset button on the thermal switches mounted on the top copper chambers.
  3. Flush the heater using vinegar to remove insulating scale buildup. Check element relays.
E4

Heating Element Failure

Warning

What it means: The unit detects that one or more heating elements are not drawing electricity when triggered, indicating an open circuit or dry fire.

Common Causes:
  • Heating element burned out due to air in the lines (Dry Fire during install).
  • Heavy mineral scale coating causing elements to crack and burn.
  • Loose wiring screw terminals on the top of the element brass base.
How to Rectify:
  1. Turn off all power breakers. Check element terminals for tight connections.
  2. Use a multimeter to measure resistance across each element's terminals. A healthy element reads ~15-18 ohms.
  3. If reading is infinite (open circuit), drain the unit, unscrew the element, and replace it.
E5

Flow Sensor Failure

Info

What it means: Water flow is detected in the pipes but the flow sensor turbine is not rotating or is sending an invalid signal.

Common Causes:
  • Debris (sand, solder flux, scale) caught in the flow sensor paddle wheel.
  • Inlet filter screen clogged, restricting flow below 0.3 GPM.
  • Flow sensor plug loose on control board.
How to Rectify:
  1. Clean the inlet filter screen at the cold water inlet joint.
  2. Ensure flow rates meet 0.3 GPM by checking fixture flow rate.
  3. Inspect and clean the internal flow sensor turbine wheel. Replace flow sensor assembly if damaged.
No Display

No Power / Dead Display

Danger

What it means: The screen is completely blank, and the unit fails to heat any water at all.

Common Causes:
  • One or more of the three double-pole 40A breakers tripped.
  • Control board transformer fuse is blown.
  • Loose ribbon cable connecting control board to front display interface.
How to Rectify:
  1. Check your home breaker panel. Fully switch OFF all three RTEX-24 double-pole breakers and turn them back ON.
  2. Open front cover (power off) and inspect display ribbon cable contacts.
  3. Check control board fuse. Replace fuse or board if power is present but display remains dead.