4 flashes = High-temperature lockout. The ECO (Emergency Cutoff Off) tripped at approximately 180°F tank shell temperature. Same diagnostic as State 4-flash codes.
Why this is a safety concern
The ECO is a one-shot safety preventing scald water (180°F+ at the tap) and steam explosion. Repeated trips indicate a real fault — don\'t just keep resetting without diagnosing.
Diagnostic sequence
1. Verify the lockout
Wait 30 minutes for the tank to cool. Attempt reset. If trips again immediately, you have an active fault. If it stays cleared, monitor over the next 24 hours.
2. Stuck thermostat (most common)
The main thermostat (integrated into the gas valve on Reliance) stuck in the closed position. Burner over-fires; ECO trips at the safety limit. Diagnosis: feel the outlet pipe immediately after a fire cycle — if scalding (well above setpoint), the thermostat is failing. Dealer replacement under warranty; non-warranty replacement is $100-180 part plus labor.
3. Heavy sediment buildup
Sediment insulates the temperature sensor from actual water temperature. Burner thinks water is cold and keeps firing. Flush the tank until water runs clear. Resolves 20-30% of 4-flash cases.
4. Low water level
If the tank wasn\'t fully refilled after maintenance, the partial water column overheats quickly. Verify the tank is completely full — open a hot tap, wait for steady stream (no air sputter), then close.
5. Thermostat sensor displaced
The sensor probe must be in firm contact with the tank shell. If accidentally pulled back during anode rod work, it reads ambient air instead of tank temperature.
Reset procedure
- Address the suspected cause
- Verify tank is full (open hot tap, no air sputter)
- Cycle gas valve to "Off" for 5 minutes
- Restart per manual
Bottom line
Most 4-flash cases: stuck thermostat (warranty replacement) or sediment (flush). Don\'t repeatedly reset without diagnosing — the ECO is a safety device protecting against scald and steam events.