The AO Smith Error Code E90 indicates a “Condensate Drain Blockage.” In high-efficiency condensing water heaters, water vapor is cooled into liquid condensate. If this liquid cannot exit through the drain line due to a clog or improper installation, the unit shuts down to prevent internal flooding and electrical damage.
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Listen, if you’re staring at that flashing “E90” on your display, you’re likely dealing with ice-cold showers and perhaps a strange gurgling sound coming from the unit. It’s frustrating, but here’s the good news from someone who’s been in the trenches: this is rarely a “broken” heater. It’s usually just a plumbing “traffic jam.” With a little patience and the right steps, we can get that water flowing and the burner firing again.
Symptoms of a Blocked Condensate Drain
When your AO Smith unit throws an E90, it doesn’t just quietly quit. You’ll notice the control panel flashing the E90 code, often accompanied by an audible alarm or beep to grab your attention. Because the unit’s safety sensors have detected backed-up water, the burner will be disabled, resulting in lukewarm or completely cold water.
If you stand near the unit, you might hear gurgling or splashing noises inside the combustion chamber or the exhaust vent—that’s the sound of the heater trying to “breathe” through standing water. In some cases, you might even see water dripping from the bottom of the cabinet or pooling around the base of the heater if the backup has found a secondary exit. If the unit attempts to fire but cuts out immediately, that’s the pressure switch telling the system it’s unsafe to operate.
How to Fix AO Smith Error E90 (Step-by-Step)
Step 1: Safety First and Power Down. Before you touch anything, turn off the power to the water heater at the circuit breaker or unplug it. Then, shut off the gas supply valve. We’re going to be dealing with water and electricity in close proximity; don’t skip this step. Let the unit sit for 10 minutes to ensure any residual heat has dissipated.
Step 2: Inspect the External Drain Line. Follow the PVC or flexible tubing from the unit to where it terminates (usually a floor drain or through a wall to the outdoors). Look for obvious kinks or sags. If the pipe goes outside and it’s winter, check the exit point for ice. Use a hair dryer (not a heat gun!) to gently melt any ice blockages. Warning: Never use a torch on PVC pipes; you will melt the pipe and potentially start a fire.
Step 3: Clear the Line with a Wet/Dry Vac. This is the “pro secret.” Go to the end of the condensate drain line. Wrap a wet rag around the nozzle of your shop vac and the end of the drain pipe to create an airtight seal. Turn the vacuum on for 30 seconds. This will usually suck out any sludge, dead bugs, or debris that’s clogging the line. Check your vacuum canister to see what came out; if it’s full of “gunk,” you’ve likely found your problem.
Step 4: Open the Cabinet and Clean the Internal Trap. Use your Phillips head screwdriver to remove the front cover of the AO Smith unit. Locate the internal condensate trap—it’s usually a clear or black plastic reservoir. Carefully disconnect the hoses (keep your bucket handy, as water will spill). Flush the trap with warm water and a little white vinegar to dissolve any mineral buildup. Ensure the internal “ball” or float moves freely.
Step 5: Check the Condensate Neutralizer. If your system has a neutralizer kit (a large horizontal tube), unscrew the caps. If the media inside looks like gray mush rather than white stones, it needs to be replaced. For a quick fix, you can temporarily bypass it to see if the error clears, but you must replace the media immediately to prevent the acidic water from eating your floor drain.
Step 6: Reassemble and Reset. Put the hoses back on, tighten all clamps, and replace the front cover. Turn the gas back on, then restore power. The unit should run a self-diagnostic. If the E90 persists, you may need to hold the “Power” or “Reset” button on the interface for 5 seconds to clear the logic board’s memory.
- Difficulty: Intermediate (Requires basic plumbing knowledge)
- Estimated Time: 45 to 90 minutes
- Tools Needed: Phillips head screwdriver, Wet/Dry shop vacuum, a flexible pipe brush or “snake,” and a small bucket.
- Estimated Cost: $0 – $40 (depending on if you need cleaning supplies or a new condensate neutralizer).
Why is my AO Smith showing Error E90?
After decades of turning wrenches, I can tell you that E90 isn’t a “parts failure” as much as it is a “maintenance failure.” Here are the three most common culprits behind that blockage:
1. Biofilm and Debris Accumulation: High-efficiency heaters produce mildly acidic condensate. Over time, dust from the air intake mixes with this moisture to create a thick, snot-like sludge known as biofilm. This “gunk” settles in the traps and elbows of your PVC drain line, eventually creating a plug that the gravity-fed system can’t push through.
2. Frozen Exterior Lines: If you live in a climate where the mercury drops below freezing, your condensate exit pipe is a prime target. Since the water trickles out slowly, it can freeze layer by layer at the exit point until the entire pipe is a solid ice spear. When the water has nowhere to go, it backs up into the heater’s internal collector, triggering the E90 code.
3. Improper Pipe Sloping or “Trap” Issues: Physics doesn’t take days off. Condensate lines must have a consistent downward slope (usually 1/4 inch per foot). Over time, support brackets can loosen, causing the pipe to “sag.” This creates a secondary trap where water sits, catches debris, and eventually blocks the flow. Additionally, if your unit uses a condensate neutralizer (a tube filled with rocks to balance pH), those rocks can break down into a fine silt that acts like cement, stopping the water cold.
How to Prevent Error E90
You don’t want to be doing this again in six months. First, perform an annual vinegar flush. Once a year, pour a cup of distilled white vinegar into the condensate clean-out port (if your plumber installed one). This kills the bacteria that creates that “snotty” biofilm and keeps the lines clear.
Second, check your pipe supports. Ensure your drain line is strapped every few feet so it doesn’t sag. A sagging pipe is a clogging pipe. Lastly, if your drain line exits the house, insulate the outdoor portion or upgrade to a larger diameter pipe for the exterior run. This prevents “flash freezing” during extreme cold snaps, keeping your heater running when you need hot water the most.
Frequently Asked Questions
Q: Can I run my heater while the E90 code is active?
A: No. The E90 code is a hard lockout. The system’s computer has disabled the burner because it senses a risk of water backing up into the combustion chamber. Forcing the unit to run or constantly resetting it without fixing the drain can lead to expensive damage to the heat exchanger.
Q: Why does this error happen more often in the winter?
A: Two reasons: First, the temperature difference between the exhaust and the outside air is greater, causing the unit to produce more condensate. Second, and most commonly, the exterior portion of the drain line freezes, creating a physical plug of ice that prevents water from exiting.
Q: My drain line is clear, but I still see E90. What now?
A: If the lines are dry and clear, you might have a faulty condensate high-limit switch or a failed sensing probe. At this point, you’ve done the DIY work; it’s time to call a tech to test the continuity of the sensors and the control board logic.