Troubleshooting AO Smith Error Code E87: What It Means & How to Fix

The AO Smith Error Code E87 is a critical diagnostic fault indicating a Pressure Switch Error. This occurs when the water heater’s integrated control system detects that the pressure switch—a vital safety component—has failed to close or open at the appropriate stage of the ignition sequence, preventing the burner from firing to ensure hazardous exhaust gases are properly vented.

When this error strikes, your household will likely experience an immediate loss of hot water, leaving you with nothing but cold or lukewarm streams. You may hear the exhaust blower motor engage and run continuously, but the unit will fail to produce the “click” of the igniter or the roar of the flame. While seeing an error code on a sophisticated appliance can be daunting, rest assured that E87 is a common mechanical or pneumatic issue that is entirely resolvable with systematic troubleshooting and the right technical approach.

Symptoms of AO Smith Error E87

As a senior engineer, I look for “behavioral signatures” of a failing unit before even cracking the case. If your AO Smith unit is throwing an E87 code, you will likely observe the following performance anomalies:

• The Lockout Cycle: Upon a call for heat, the exhaust blower (inducer) will spin up, attempting to clear the combustion chamber and create the necessary pressure differential. After 30–60 seconds of high-speed rotation without ignition, the unit will shut down and display the E87 code.
• Audible Component Failure: You might hear a “chattering” sound coming from the pressure switch as the internal diaphragm struggles to maintain a steady contact, or conversely, a total lack of a “click” when the fan reaches full speed.
• Flashing Display: The LED or LCD interface will persistently flash “E87” or a sequence of light pulses (refer to your specific model’s blink code chart) that translate to the pressure switch circuit.
• Environmental Feedback: In some cases, you may notice a slight smell of unburned gas or see moisture/condensate backing up in the pressure sensing tubes, which is a secondary symptom of the primary venting issue causing the E87.

Troubleshooting & Replacement Instructions

Troubleshooting & Replacement Instructions

Follow these steps precisely. As an engineer, I emphasize that safety is paramount. Failure to follow electrical safety protocols can result in injury or damage to the unit’s sensitive logic boards.

Step 1: Isolation and Power Down
Before touching any internal components, turn off the gas supply valve to the water heater. Locate the power cord or the dedicated circuit breaker and disconnect power. Wait at least 60 seconds for any residual electrical charge in the capacitors to dissipate. WARNING: Working on a live unit can result in fatal electric shock or short-circuiting the expensive main controller.

Step 2: Access the Control Compartment
Using your Phillips head screwdriver, remove the screws securing the front access panel. Carefully lift the panel off and set it aside. Locate the pressure switch; it is usually a circular or rectangular plastic component with two wires and one or two translucent plastic tubes attached to it, mounted near the exhaust blower motor.

Step 3: Inspect Pneumatic Tubing
Gently pull the plastic tubing off the pressure switch and the blower housing. Inspect the ends for cracks or “dry rot.” Blow through the tubes to ensure they are clear of moisture or debris. If you find water inside, the venting may be improperly sloped, which will cause the E87 to return until the venting is corrected.

Step 4: Electrical Continuity Test
Set your multimeter to the Ohms (Ω) or Continuity setting. Remove the two wire leads from the pressure switch. With the fan OFF, the switch should read “Open” (OL or infinite resistance). If it shows continuity while the fan is off, the switch is “stuck closed” and must be replaced. Next, while the unit is briefly powered on and the fan is running, the switch should “close” and show 0.0 to 0.5 Ohms. If it remains “Open” while the fan is at full speed, the switch is defective.

Step 5: Inspect the Venting Termination
Go outside to where the water heater vents through the wall or roof. Ensure there are no obstructions such as leaves, insect nests, or snow. Even a partial blockage can prevent the pressure switch from reaching its “trip point.” Clear any debris found and ensure the screens are clean.

Step 6: Replacing the Switch
If the switch failed the multimeter test, unscrew the mounting bracket. Install the new, OEM-specified AO Smith pressure switch. Reattach the pneumatic tubes, ensuring a snug, airtight fit. Reconnect the wire leads to the appropriate terminals. Note: Always use original manufacturer parts, as pressure switches are calibrated to specific “Inches of Water Column” (WC) ratings unique to your model.

Step 7: System Re-initialization
Replace the front cover, restore the gas supply, and plug the unit back in. The control board should undergo a self-test. Initiate a call for hot water by turning on a faucet. The blower should kick on, followed by the “click-click-click” of the igniter, and finally a steady flame. The E87 code should now be cleared.

Detailed Diagnosis: Root Causes

Detailed Diagnosis: Root Causes

The pressure switch is a “normally open” safety device designed to prove that the inducer fan is moving enough air to safely vent carbon monoxide. When the E87 error triggers, it means the circuit logic is broken. Here are the primary engineering reasons for this failure:

1. Mechanical Diaphragm Failure: Inside the pressure switch is a sensitive silicone or rubber diaphragm. Over years of operation, the constant expansion and contraction can cause the material to stiffen, crack, or lose its elasticity. If the diaphragm cannot flex sufficiently under the vacuum produced by the blower, the micro-switch will never “make” the circuit, leading to a permanent E87 lockout.

2. Pneumatic Obstructions (Vent Blockage): This is often an external environmental issue rather than a component failure. If birds build a nest in the PVC vent termination, or if ice buildup occurs during winter, the static pressure within the vent pipe changes. The switch detects that the airflow is insufficient for safe combustion and refuses to let the gas valve open. This is a safety feature, not a bug.

3. Tubing Contamination and Degradation: The small, flexible plastic tubes connecting the pressure switch to the blower housing are prone to collecting condensate (water) or debris. Even a single drop of water trapped in the tube can act as a plug, preventing the switch from “sensing” the pressure. Furthermore, heat from the unit can cause these tubes to become brittle and crack, leading to vacuum leaks.

4. Voltage Irregularities or Control Board Logic Errors: Occasionally, the E87 is a ghost in the machine caused by a voltage spike that has damaged the 24V DC circuit on the main control board. If the board cannot “read” the return signal from a perfectly functional switch, it defaults to an E87 error for safety reasons.

How to Prevent Error E87

Proactive maintenance is the hallmark of a well-run mechanical system. To ensure you never face a cold shower due to an E87 error again, implement these engineering-standard practices:

• Annual Vent Inspections: At least once a year, visually inspect the entire length of your intake and exhaust venting. Look for sagging pipes where condensate could pool and block airflow. Ensure the outdoor terminations are clear of seasonal debris or vegetation growth.
• Clean the Intake Screen: Many AO Smith power-vented units have a small screen or filter on the air intake. If this becomes clogged with dust or pet hair, the blower won’t be able to pull enough air to activate the pressure switch. Clean this monthly in high-dust environments.
• Install a Power Surge Protector: The control boards on modern water heaters are highly sensitive to “dirty” power. A dedicated surge protector can prevent voltage spikes from frying the logic circuits that monitor the pressure switch, extending the life of your electronics.

Frequently Asked Questions

Q: Can I bypass the pressure switch just to get a hot shower tonight?
A: Absolutely not. As an engineer, I must stress that the pressure switch is a life-safety device. Bypassing it allows the unit to fire without confirmed venting, which can lead to Carbon Monoxide (CO) poisoning inside your home. Never jump or bypass safety sensors.

Q: My fan is spinning, so why does the switch say there’s no pressure?
A: A motor “spinning” doesn’t always mean it’s moving enough air. The blower wheel inside the housing could be coated in dust or have broken fins, reducing its efficiency. Alternatively, the “port” where the tube connects to the blower might be clogged with a tiny bit of corrosion or debris, preventing the vacuum signal from reaching the switch.

Q: Is Error E87 different from Error E88?
A: Yes, though they are related. While E87 generally refers to the switch failing to close or open when expected during the cycle, other codes in the 80-series might refer specifically to “high limit” issues or “fan speed” errors. Always check your specific model’s manual, but E87 is almost universally the pneumatic pressure circuit.

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