Reliance Water Heater Not Working: Diagnosis and Repair Guide

Common Symptoms

• Reliance water heater produces no hot water at all — tank completely cold
• Reliance 6-Series or 9-Series gas: pilot will not light or won't stay lit after holding
• Reliance gas: status indicator light (SIL) on gas valve is blinking a fault code
• Reliance gas: pilot lights but main burner never fires when hot water is demanded
• Reliance electric 12-Series: circuit breaker tripped — or breaker on but no heat
• Reliance electric: pressing the breaker reset does nothing — unit still stone cold
• Reliance 9-Series power vent: power vent fan runs but ignition does not occur
• Loud sediment rumbling followed by complete loss of heat — thermal switch tripped

Most Likely Causes

1. 1

   Reliance Gas: Thermocouple or Thermopile Failure — Pilot Won't Hold

   The leading cause of a Reliance gas water heater stopping completely is a failed thermocouple or thermopile. Reliance 6-Series atmospheric vent models use either a traditional thermocouple (generates 17–35mV) or a thermopile (generates 300–750mV). The gas valve requires a minimum voltage from the thermocouple or thermopile to hold the gas valve open — below that threshold, the valve shuts off all gas as a safety measure. Common Reliance thermocouple part: 9003972 (AO Smith/Reliance OEM). Common thermopile: 9005502. A worn thermocouple reads below 17mV on a millivolt meter. A thermopile reading below 325mV while the pilot is lit indicates it needs replacement. The status indicator light (SIL) on Reliance gas valves will flash 1 time for low thermopile voltage.

2. 2

   Reliance Gas: Status Indicator Light (SIL) Blink Codes

   Reliance gas water heaters with the Honeywell WV8840 or similar combination gas valve display diagnostic blink codes via the status indicator light (SIL) — a small LED or lens visible through the gas valve cover. Count flashes per burst, then the pause, then the next burst. Key codes for Reliance models: 1 flash = thermopile voltage too low (pilot lit, thermopile output below 325mV — weak thermocouple/thermopile); 2 flashes = thermal switch (TCO) open — a safety switch tripped due to overheating, blocked flue, or sediment; 3 flashes = gas valve internal fault; 4 flashes = gas valve temperature too high (blocked or restricted flue vent); 7 flashes = gas valve failed and must be replaced. Steady on / continuous blinking = normal operation (pilot lit, valve ready to fire main burner). No light and no pilot = no thermocouple voltage reaching valve.

3. 3

   Reliance Gas: Thermal Cutout (TCO) Tripped — 2-Flash Code

   Reliance gas water heaters have a high-temperature thermal cutout (TCO) mounted on the gas valve or burner bracket assembly. If the burner area exceeds safe temperature limits — caused by a blocked vent flue, restricted air inlet, excessive sediment insulating the tank bottom, or multiple consecutive short-cycle ignitions — the TCO opens permanently and the SIL flashes 2 times. The unit will not fire until the TCO is manually reset. Unlike a circuit breaker, the TCO will trip again immediately if the underlying cause is not addressed first. The Reliance 9-Series power vent models also have a thermal limit switch on the blower motor housing — if the blower overheats or fails, this switch opens and prevents burner ignition.

4. 4

   Reliance 9-Series Power Vent: Blower or Ignition Control Failure

   Reliance 9-Series direct vent and power vent gas water heaters require 120V AC power for the blower motor and electronic ignition module. A tripped GFCI outlet upstream, a failed blower motor (capacitor failure is common), or a failed ignition control board will prevent ignition even if the gas supply and thermocouple are fine. The SIL will blink 4 times if the blower fails to establish adequate draft pressure. The pressure switch on 9-Series models monitors draft — if the blower runs but the pressure switch does not close, ignition is locked out. Check: (1) Is the 120V outlet powering the unit working? (2) Does the blower spin freely when power is applied? (3) Is the vent pipe clear of bird nests, condensate ice, or debris?

5. 5

   Reliance Electric 12-Series: ECO Thermal Cutout Tripped

   Reliance electric water heaters (12-Series prefix, e.g. 12-50-NOLSM) use a dual-element, dual-thermostat design with a manual-reset ECO (Energy Cut-Out) safety switch on each thermostat. If the water temperature exceeds approximately 180°F due to thermostat runaway or an element shorted to ground, the ECO trips and cuts power to all heating elements. The circuit breaker at the panel will NOT trip — the unit appears on but does not heat. To find the ECO: remove both access panels on the side of the tank, fold back the foam insulation, and look for the small red or white push-button on the face of each thermostat body. If either button is popped out, it has tripped. Press firmly until it clicks. If it trips again within the next heating cycle, the thermostat has failed and must be replaced.

6. 6

   Reliance Electric 12-Series: Burned-Out Heating Element

   A burned-out heating element on a Reliance electric model is a common cause of the unit producing no hot water (upper element failure) or inadequate hot water (lower element failure). Elements fail as either open circuits (no heat, no breaker trip) or shorted to ground (trips the 240V breaker when heating begins). Reliance electric models typically use 4500W screw-in elements at 240V. Diagnose with a multimeter: with the 240V breaker OFF, disconnect element leads and measure resistance terminal-to-terminal (healthy: 12–16 ohms for 4500W) and terminal-to-sheath (should be infinite — any low reading indicates a ground fault). Replace with Camco 02142 (4500W) or the OEM element for your Reliance model.

Quick DIY Checks

1. 1Step 1 — Reliance gas: read the status indicator light (SIL) blink code: locate the gas valve on your Reliance water heater — it is the metal control assembly with the temperature dial and PILOT/ON/OFF knob, typically at mid-height on the tank front. The status indicator light (SIL) is a small LED window or lens on the valve face. Count the flashes per burst carefully: 1 flash = low thermopile voltage; 2 flashes = TCO tripped; 3 flashes = gas valve fault; 4 flashes = gas valve overtemp or blower issue (9-Series); 7 flashes = valve failed. Steady blinking = normal. No light and pilot out = no thermocouple/thermopile voltage. Write down the code — it tells you exactly where to focus. If there is no SIL light and you see the pilot flame, the system may be in normal standby and the call for heat is the issue.
2. 2Step 2 — Reliance gas: relight the pilot and test thermocouple output: turn the gas valve knob to PILOT. Press and hold the knob in (or depress the red piezo button on the side) while pressing the igniter button repeatedly until the pilot flame lights. Hold the knob depressed for 45–60 full seconds to heat the thermocouple. Slowly release — if the pilot extinguishes, the thermocouple is below the minimum threshold. To measure: with the pilot lit, connect a DC millivolt meter to the TH/TP and TH terminals on the gas valve (small spade or screw terminals on the valve body). Healthy thermocouple: 25–35mV. Minimum: 17mV. Thermopile: 400–750mV healthy, 325mV minimum. Replace with Reliance/AO Smith 9003972 (thermocouple) or 9005502 (thermopile) for models with thermopile.
3. 3Step 3 — Reliance gas: investigate and reset a tripped TCO (2-flash SIL code): a 2-flash code means the thermal cutout (TCO) has opened due to overheating. Before resetting, identify the cause: check that the vent flue pipe above the draft hood is clear of obstructions; verify the air inlet at the base of the tank is not blocked by stored items; listen for sediment rumbling (heavy sediment insulates the tank bottom, causing local overheating). On Reliance gas models, the TCO reset is part of the gas valve assembly — turn the knob to OFF, wait 5 minutes for cooling, then follow the relighting procedure. On some models, the burner access door at the bottom of the tank has an accessible reset button on the burner bracket — press firmly. If the TCO trips again, the flue or combustion air supply must be addressed before the heater is safe to operate.

Repair vs Replace

Recommended Tools & Parts

• Reliance/AO Smith Thermocouple 9003972

  OEM thermocouple for Reliance gas water heaters (thermocouple-based 6-Series models). 36-inch lead. Replace when pilot won't hold after 45 seconds of button hold, or millivolt test shows below 17mV. Interchangeable with AO Smith and State ProLine units.

  $12–$20

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• Reliance/AO Smith Thermopile 9005502

  OEM thermopile for Reliance gas models with thermopile (1-flash SIL code models). Generates 300–750mV. Replace when SIL shows 1-flash and thermopile millivolt test is below 325mV with pilot lit.

  $28–$45

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• Camco 02142 4500W Screw-In Heating Element

  Universal screw-in 4500W 240V element for Reliance electric 12-Series water heaters. Replace when open circuit (OL terminal-to-terminal) or ground fault (low resistance terminal-to-sheath) confirmed on multimeter test. Drain tank fully before removal.

  $18–$35

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• Reliance/AO Smith Thermostat Kit 100110321

  Replacement upper thermostat with ECO cutout for Reliance electric 12-Series models. Replace when ECO trips repeatedly within a single heating cycle despite element testing good — indicates thermostat runaway.

  $22–$40

  Buy on Amazon →

Links are Amazon affiliate links (tag: fixitfastai-20). Prices are estimates.

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