State Water Heater E121 Error — What It Means
Code E121 on a State tankless water heater (which uses A.O. Smith technology) indicates an ignition failure. The control system attempted to ignite the burner three times but detected no flame within the preset ignition window. The unit then enters a safety lockout to prevent unsafe conditions.
This lockout is a protective measure. The heater will not fire again until you resolve the underlying issue and manually reset the system by turning off the gas and water supply, waiting five seconds, and turning both back on.
Before You Replace Anything
Homeowners often assume the gas control valve has failed and replace it first. Before ordering that expensive part, check the air intake screens for lint or spider nests and verify the gas line size is at least ¾ inch to the unit.
Common Causes
- Clogged air intake screens (~35%) Lint, debris, or spider nests block the exterior air intake screens and starve the burner of combustion air.
- Blocked vent or flue (~25%) Bird nests, debris, or improper installation of the double-walled concentric flue prevent exhaust gases from venting and disrupt ignition.
- Insufficient gas supply (~20%) Gas line too small (needs ¾ inch minimum), low inlet pressure, or air trapped in the line after installation prevents fuel from reaching the burner.
- Dirty flame rod sensor (~12%) Scale or carbon buildup on the flame sense rod prevents the control from detecting ignition even when a flame is present.
- Failed gas control valve (~5%) The valve does not open despite receiving the correct signal from the control board, blocking gas flow to the burner.
- Low water flow (~3%) A clogged flow sensor or insufficient flow rate prevents the unit from firing within the ignition timeout window.
Quick Diagnosis
Answer these to narrow it down fast.
Do you see or hear a spark at the burner when you call for hot water?
No: No spark means an electrical issue with the ignition system or control board, or the unit is in lockout and needs a manual reset.
Are the air intake screens clean and free of lint or nests?
No: Clean the screens thoroughly and reset the unit by turning off gas and water for five seconds, then turning both back on.
Is the gas line to the unit at least ¾ inch in diameter?
No: Upgrade the gas line to ¾ inch minimum to meet the BTU demand of the tankless unit.
Step-by-Step Fix
- Turn off the gas and water supply to the unit and wait five seconds to reset the lockout.
- Remove and clean the air intake screens on the exterior of the unit, checking for lint, spider webs, or other debris.
- Inspect the vent flue for blockages, bird nests, or improper installation, and confirm it is the correct A.O. Smith double-walled concentric flue with a maximum length of 42 feet.
- Verify the gas line size from the main supply to the unit is at least ¾ inch in diameter, and bleed any trapped air from the line if the unit was recently installed or the gas was shut off.
- Check inlet gas pressure using a manifold gauge (should be 5 to 7 psi for natural gas), and adjust the regulator or call your gas utility if pressure is low.
- Clean the flame rod sensor with fine emery cloth to remove scale or carbon buildup that prevents flame detection.
- Turn the gas and water back on and test for ignition, observing whether the unit sparks and establishes a flame within a few seconds.
Parts Often Needed
| Part | Notes |
|---|---|
| Flame rod sensor (State / A.O. Smith tankless) | Amazon | Order by your model number if cleaning does not restore function. |
| Gas control valve (State / A.O. Smith tankless) | Amazon | Replace only after confirming gas supply, pressure, and flame rod are all good. |
| Double-walled concentric flue kit (A.O. Smith) | Amazon | Must be the correct A.O. Smith part number for your model and installation length. |
| Flow sensor (State / A.O. Smith tankless) | Amazon | Replace if water flow is insufficient or the sensor is clogged with debris. |
When to Call a Pro
Call a licensed plumber or gas technician for any E121 code. Tankless water heaters involve natural gas or propane, high-voltage ignition systems, and precise gas-pressure settings that require professional tools and training. A technician will measure inlet gas pressure with a manifold gauge, inspect the double-walled concentric flue for code compliance, and test the flame rod and gas control valve with a multimeter. If you smell gas, leave the area immediately and call your gas utility or fire department before attempting any diagnostics.
Rough cost: A pro service call runs about $150-350.