Rheem 5 Flashes: Gas Valve Sensor Fault Diagnosis

A Rheem gas water heater’s 5-flash error indicates a failed thermistor. This guide covers diagnosing the open/shorted sensor circuit and replacing the valve.

Deep Technical Diagnosis: The Physics of the Failure

A 5-flash diagnostic code on a Rheem gas control valve indicates a definitive failure within the water temperature sensing circuit. The core of this circuit is a Negative Temperature Coefficient (NTC) thermistor, a semiconductor whose electrical resistance decreases predictably as temperature increases. The unit’s microprocessor sends a reference voltage through the thermistor and measures the resulting signal in millivolts to calculate the precise water temperature. A ‘5 flash’ error is triggered when the control board detects a resistance reading that is critically out of its expected range—either near-infinite resistance (an open circuit) or near-zero resistance (a short circuit).

An open circuit typically occurs when the delicate internal wiring of the thermistor fractures due to thermal stress cycles or vibration. The control board registers this as an infinite impedance path, which is an impossible temperature reading, initiating an immediate safety lockout. Conversely, a short circuit can be caused by internal component failure or, more insidiously, by water ingress past a failing seal, creating a conductive path that bypasses the resistive element. This low-resistance reading would be interpreted as an extremely high, unsafe temperature, also triggering a hard lockout. Before total failure, the thermistor may provide erratic readings, causing fluctuations in burner operation as the control attempts to maintain a setpoint based on faulty data. This can negatively impact the unit’s ability to achieve proper stoichiometric combustion, potentially leading to increased carbon monoxide production. A technician would use a multimeter set to ohms (Ω) to test the thermistor leads (with power off); a healthy sensor at room temperature (~77°F) should read approximately 10,000 ohms (10kΩ). A reading of ‘O.L.’ (Over Limit) confirms an open circuit, while a reading near 0Ω confirms a short.

• Internal Wire Fracture: The most common cause, where repeated expansion and contraction from heating cycles physically breaks the internal filament of the NTC thermistor.
• Seal Failure and Water Intrusion: The seal around the thermistor probe degrades, allowing tank water to penetrate the sensor housing, causing a direct electrical short.
• Connector Corrosion: Oxidation or moisture at the Molex connector between the thermistor leads and the main control board can create excessive resistance, mimicking an open circuit.
• Control Board Fault: In rare cases, the analog-to-digital converter on the control board itself fails, preventing it from correctly interpreting the signal from a perfectly functional thermistor.
• Calcification Bridging: Severe mineral scale can build up and physically damage the sensor or create an alternative electrical path on the exterior of the probe, leading to erroneous readings.

Professional Master Plumber Repair Sequence

1. Safety Protocol Initiation: Turn the gas control valve knob to the ‘OFF’ position. Locate the main gas shutoff valve on the supply line to the water heater and turn it to the closed position (typically a quarter turn so it is perpendicular to the pipe). Shut off the cold water supply valve to the tank.
2. System Depressurization and Draining: Open a hot water faucet somewhere in the building to relieve pressure. Connect a garden hose to the drain valve at the bottom of the tank and route it to a floor drain or outside. Open the drain valve and the T&P valve to allow air in and facilitate a full drain. This will take 30-60 minutes.
3. Component Disconnection: Carefully disconnect the burner assembly door and any electrical connections to the gas control valve, such as the piezo igniter wire and any sensor harnesses. Note their locations. Using two pipe wrenches (one for backup to prevent stressing the tank connections), carefully disconnect the main gas supply line from the control valve.
4. Remove Old Gas Control Valve: The gas control valve is threaded directly into the tank spud. Using a large pipe wrench, carefully unthread the entire valve assembly from the tank by turning it counter-clockwise. Be prepared for some residual water to spill out.
5. Prepare Tank Threads: Once the old valve is removed, inspect the threads on the tank spud. Clean them thoroughly with a wire brush to remove any old pipe dope, tape, and corrosion. Wipe the threads clean with a rag.
6. Install New Gas Control Valve: Apply a moderate amount of gas-rated Teflon tape or pipe joint compound to the male threads of the new gas control valve ONLY. Carefully thread the new valve into the tank spud by hand to avoid cross-threading. Tighten it with a pipe wrench until it is snug and properly oriented (typically 1-2 full turns past hand-tight). Do not overtighten.
7. Reconnect All Lines: Reconnect the burner manifold tube, main gas supply line, piezo igniter, and any other electrical harnesses to the new control valve. Ensure all connections are secure. Use a backup wrench when tightening the gas line fitting.
8. Refill the Tank: Close the drain valve. Open the cold water supply valve to begin refilling the tank. Keep the hot water faucet open elsewhere in the house; once water flows steadily from it, the tank is full. Check for any water leaks around the new valve’s threads.
9. Gas Leak Test and Commissioning: Open the main gas shutoff valve. Using a gas leak detection solution, coat every joint and fitting you disturbed. Look for bubbles, which indicate a gas leak. If a leak is found, shut off the gas and tighten the connection. Once confirmed leak-free, follow the manufacturer’s specific lighting instructions printed on the heater to relight the pilot and return the unit to service.