Troubleshooting AO Smith Error Code E86: What It Means & How to Fix

What is AO Smith Error E86?

The AO Smith Error E86 signifies a “High Limit Switch Open” condition. Specifically, the Energy Cut Off (ECO) safety device has detected a temperature exceeding the safe operating threshold (typically above 170°F to 180°F) and has physically broken the electrical circuit to prevent a catastrophic tank failure or severe scalding. This is a critical safety deployment.

If you are seeing this code, you are likely experiencing a total loss of hot water, or perhaps you noticed the water was dangerously hot just before the unit shut down. You might also hear an audible “click” or see the display flashing red. While this indicates a serious safety shutdown, it is a fixable issue that often involves resetting a switch or replacing a faulty sensor.

Symptoms of Error E86

As a safety officer, I must emphasize that ignoring these symptoms can lead to hazardous conditions. If your AO Smith water heater is displaying Error E86, you will observe one or more of the following:

• Control Panel Alert: The digital display will explicitly flash “E86” or “High Limit Open,” often accompanied by a red LED warning light.
• Complete Lack of Hot Water: Since the ECO switch has broken the circuit, the heating elements will not receive power, resulting in strictly cold water from your faucets.
• Water Temperature Spikes: Just prior to the error occurring, you may have noticed the water coming out of the tap was significantly hotter than your thermostat setting.
• Reset Button Tripping: You may find the physical red reset button on the upper thermostat has popped outward, indicating a thermal trip.
• Audible Hissing or Popping: In cases of extreme overheating, you may hear the water inside the tank boiling or popping due to sediment interference.

Why is my AO Smith showing Error E86?

The E86 code is not a random malfunction; it is a response to a specific physical or electrical stimulus. Understanding the root cause is vital for a permanent repair.

• Faulty Upper or Lower Thermostat: Thermostats are mechanical or electronic “gatekeepers.” If a thermostat becomes stuck in the “ON” position, it will continue to power the heating element indefinitely. This leads to “runaway heating,” where the water temperature exceeds the safety limit, forcing the ECO switch to open to prevent the tank from exploding.
• Heating Element Grounding: Over time, the outer sheath of a heating element can corrode and crack. This allows the internal filament to come into contact with the water or the tank. Even if the thermostat shuts off, electricity continues to flow through the water (grounding), causing the temperature to rise uncontrollably until the high limit switch trips.
• Internal Sensor/ECO Failure: Like any electrical component, the High Limit Switch itself can fail due to age, moisture, or internal wear. If the internal contacts within the switch degrade or become “pitted,” they may open prematurely even if the water temperature is within a safe range.
• Excessive Voltage or Surges: A sudden spike in the electrical grid or a lightning strike can damage the sensitive electronic control board or the thermal sensors, causing a false E86 reading or a physical trip of the safety circuit.

Comprehensive Repair Guide

SAFETY WARNING: Electric water heaters operate on 240 Volts. This level of electricity is LETHAL. You must disconnect the power at the main circuit breaker before attempting any diagnostic steps. Use a non-contact voltage tester to verify the power is off before touching any internal components.

1. Power De-Energization: Locate your home’s main electrical panel. Identify the double-pole breaker labeled “Water Heater.” Flip it to the “OFF” position. Use your non-contact voltage tester at the water heater’s junction box to confirm zero electrical activity.
2. Access the Control Panel: Use a Phillips head screwdriver to remove the upper access panel on the side of the tank. Carefully remove the fiberglass insulation (wear gloves to avoid skin irritation) to reveal the thermostat and the ECO switch.
3. Inspect for Thermal Damage: Conduct a visual inspection of the wiring. Look for “charring,” melted plastic, or discolored wires. If you see burnt components, do not attempt a reset; the parts must be replaced immediately as they represent a fire hazard.
4. Perform a Manual Reset: Locate the red button on the ECO/Thermostat assembly. Firmly press the button inward. If you hear or feel a “click,” the switch was physically tripped due to high heat. Re-assemble the unit and restore power. If the code clears, monitor the water temperature closely. If it trips again, proceed to the next step.
5. Continuity Testing with a Multimeter: With power still OFF, set your digital multimeter to the “Ohms” (Ω) or Continuity setting. Place the probes on the terminals of the ECO switch. If the meter shows “OL” (Open Loop) despite being reset, the switch is defective and cannot pass electricity. It must be replaced.
6. Check the Heating Elements: If the switch is fine but the tank keeps overheating, test the heating elements for a “short to ground.” Place one probe on a terminal screw and the other on the metal tank. Any reading other than “OL” indicates a grounded element that is causing the overheating.
7. Component Replacement: If a component is found to be faulty, disconnect the wires (labeling them first), unscrew the mounting bracket, and install a genuine AO Smith replacement part. Ensure all terminal screws are tightened securely to prevent “arcing,” which can cause future E86 errors.

How to Prevent Error E86

Preventative maintenance is the cornerstone of safety and equipment longevity. To avoid future High Limit Switch failures, follow these protocols:

• Annual Sediment Flush: Calcium and lime buildup at the bottom of the tank can bury the lower heating element. This causes the element to overheat and creates localized “hot spots” that can trick the sensors or cause the element to fail. Flush your tank every 12 months to maintain thermal accuracy.
• Thermostat Calibration: Set your water heater temperature to 120°F. Not only is this the standard for scalding prevention, but it also reduces the thermal stress on the ECO switch, lowering the likelihood of a safety trip.
• Install a Surge Protector: Since modern AO Smith heaters use electronic control boards, installing a whole-home surge protector can shield the sensitive E86-monitoring circuitry from electrical “noise” and voltage spikes that lead to nuisance tripping.

Frequently Asked Questions

Q: Can I just bypass the ECO switch to get hot water?

A: ABSOLUTELY NOT. As a safety officer, I must state that bypassing a safety limit switch is an extreme fire and explosion hazard. The ECO is there to prevent the tank from becoming a pressurized bomb. If the switch opens, there is a dangerous condition that must be repaired, not bypassed.

Q: How much does a professional charge to fix Error E86?

A: Typically, a licensed plumber or electrician will charge between $150 and $300 for this repair. This includes a diagnostic fee, the cost of the replacement thermostat or ECO switch (usually $20-$50), and labor. Given the electrical risks, professional intervention is highly recommended.

Q: My reset button won’t stay pushed in. What does that mean?

A: If the red reset button immediately pops back out after being pressed (and the water has had time to cool down), the internal mechanical linkage of the switch has failed. The component is non-serviceable and requires total replacement of the upper thermostat assembly.