State Water Heater E91 Error — What It Means
The E91 error code on a State water heater signals a communication failure between the Electronic Control Board (the main controller) and the Interface Board (the display panel). The two microprocessors cannot exchange data, so the display cannot show status updates and the control board cannot receive user commands. This is not a sensor fault or a combustion problem. It is a digital communication breakdown between the two main circuit boards inside the unit.
Before You Replace Anything
Homeowners sometimes replace the Interface Board (display) first because it is visible and accessible. Check and reseat all wire connectors between the boards before replacing any board. The main Electronic Control Board is usually the culprit when wiring is intact.
Common Causes
- Loose or corroded wiring harness (~40%) The wires connecting the Electronic Control Board to the Interface Board are not fully seated, have corroded pins, or are physically damaged.
- Defective Electronic Control Board (~35%) The main controller board has failed and can no longer send or receive communication signals.
- Defective Interface Board (~20%) The display panel board itself has failed and cannot communicate with the main controller.
- Poor connector seating (~5%) The terminals where the boards plug together are not making solid contact even though the wires are intact.
Quick Diagnosis
Answer these to narrow it down fast.
Have you turned off power and reseated all wire connectors between the control board and the display?
No: Do that first. Power down the unit, unplug and firmly re-plug every harness connector, then restore power and check if the code clears.
Do you see any visible corrosion, burn marks, or damaged pins on the wire connectors or boards?
No: The fault is internal to one of the boards. Replace the Electronic Control Board first, then the Interface Board if the error persists.
After replacing the main Electronic Control Board, does the E91 code still appear?
No: The main board was the problem. The repair is complete.
Step-by-Step Fix
- Turn off all power to the water heater at the breaker and verify the display is dark.
- Remove the access panel covering the Electronic Control Board and the Interface Board (consult your model’s service manual for exact location).
- Inspect every wire harness connecting the two boards. Look for loose connectors, corroded pins, or damaged insulation.
- Reseat all connectors by unplugging and firmly re-plugging each one. make sure pins are clean and straight.
- Restore power and run the unit through its diagnostic cycle to see if the E91 code clears.
- If the error persists, replace the Electronic Control Board first (this is the main controller and the most common board failure for E91).
- If the error continues after installing a new main board, replace the Interface Board. If the code still appears after both boards are new, replace the wiring harness.
Parts Often Needed
| Part | Notes |
|---|---|
| Electronic Control Board (State Water Heater Main Controller) | Amazon | Match the part number on your existing board or use the model number from the rating plate to order the correct replacement. |
| Interface Board (State Water Heater Display Panel) | Amazon | Only replace if the main Electronic Control Board replacement does not clear the E91 code. |
| Wiring Harness (Control to Display) | Amazon | Replace if the existing harness shows physical damage, corrosion, or if both boards have been replaced and the error remains. |
When to Call a Pro
Call a licensed technician if you are not comfortable working with the electrical components inside a water heater. The repair requires turning off power, removing panels, and handling control boards. If you replace the wiring and both boards and the error still appears, the problem may be a deeper circuit fault or a grounding issue that requires professional diagnostic equipment. State water heaters are manufactured by A.O. Smith, and technicians have access to model-specific diagnostic procedures and genuine replacement boards. If the unit is still under warranty, professional service may be covered or required to maintain the warranty.
Rough cost: A pro service call runs about $250-450.