State Water Heater E28 Error Code — What It Means
The E28 error code on State tankless and combi water heaters (using HTP-style controls) means the control board is not detecting proper 120 VAC power supply to the gas valve. Without that power, the unit cannot safely open the valve to supply gas to the burner, so it locks out and displays E28. This is a safety feature designed to prevent unsafe operation when the gas-valve circuit is incomplete.
The code does not always mean the gas valve itself has failed. Instead, it signals that somewhere in the circuit leading to the gas valve, power is being interrupted. This can be due to loose wiring, a failed safety switch (such as a pressure switch or ECO switch), or less commonly a faulty gas valve that cannot accept power correctly.
Common Causes
- Loose or damaged wiring to the gas valve Vibration, corrosion, or improper installation can cause connectors at the gas valve or control board to loosen and break the 120 VAC circuit.
- Open exhaust pressure switch If the exhaust pressure switch fails to close (due to blockage, venting issues, or a defective switch), it will open the safety circuit and prevent gas-valve power.
- Open air intake pressure switch A stuck or failed air intake pressure switch will not allow the gas-valve circuit to complete, triggering E28 even if all other components are working.
- Tripped or failed ECO or water switch These safety switches open if the unit overheats or detects unsafe water conditions, cutting power to the gas valve until the fault is cleared.
- Faulty burner safety switch If the burner safety switch does not close properly, the control will not energize the gas valve and will display E28.
- Failed gas valve In rare cases the gas valve itself may have an internal open coil or fault that prevents it from accepting 120 VAC, but this should only be suspected after all upstream circuit checks pass.
Step-by-Step Fix
- Disconnect power at the breaker and follow lockout/tagout procedures, then shut off the gas supply to the unit.
- Inspect and reseat all wiring connections at the gas valve, control board, and each safety switch (exhaust pressure, air intake, ECO, burner) to rule out loose or corroded connectors.
- Use an ohmmeter to test continuity through each safety switch in the gas-valve circuit (exhaust pressure switch, air intake pressure switch, ECO/water switch, and burner switch) while the unit is off.
- Identify the first open circuit in the chain and determine whether the switch is mechanically stuck, out of specification, or if wiring to that switch is broken.
- Repair or replace the failed component (tighten or replace damaged wiring, or swap out the defective switch), then restore power and gas.
- Run the unit and monitor startup to confirm E28 does not return and that the gas valve receives power and opens normally.
- If E28 persists after all switches test good and wiring is intact, consult your service manual’s gas-valve test procedure or replace the gas valve as a last resort.
Parts Often Needed
| Part | Notes |
|---|---|
| Exhaust pressure switch | Amazon | Verify the exact pressure-setpoint specification for your model before ordering. |
| Air intake pressure switch | Amazon | Match the switch type (normally open or normally closed) to your unit’s wiring diagram. |
| Gas valve (120 VAC) | Amazon | Only replace after confirming all upstream wiring and safety switches are intact and functional. |
| Wiring harness or connector kit | Amazon | Useful if corrosion or damage is found at gas-valve or switch terminals. |
When to Call a Pro
Call a licensed technician if you are not comfortable working with 120 VAC circuits or natural-gas appliances. Gas-fired water heaters require proper venting, combustion-air supply, and safety-switch setup. Misdiagnosis or incorrect wiring can create fire or carbon-monoxide hazards. A qualified pro will have the meter, manufacturer training, and parts access to isolate the open circuit quickly and restore safe operation. If your unit is still under warranty, professional service may be required to preserve coverage.