In my thirty years of turning wrenches, I’ve seen this play out the same way every time: you’re jumping into a shower expecting steam, but you get hit with a face full of ice water. You might see the display screen flashing that “E05” code like a neon sign, or the unit might just stop responding entirely. Don’t panic and go shopping for a new heater yet—this is a localized component failure, and it’s one of the most straightforward repairs you can do yourself.
Symptoms of a Failed Lower Thermistor
When your AO Smith unit throws an E05, it doesn’t usually happen in a vacuum. You’ll notice a few specific physical signs that the lower sensor has gone south. First and foremost is the lukewarm water syndrome. Because the lower element isn’t firing, only the top third of your tank is getting heated, leading to very short hot showers.
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You might also hear a faint clicking sound coming from the control board as it attempts to engage the lower heating circuit, only to be blocked by the shorted sensor data. Most obviously, the digital display will lock on the E05 code, and the “Status” light will likely blink in a specific red-light sequence. If you try to reset the breaker, the code usually reappears within minutes of the unit attempting a heating cycle.
How to Fix AO Smith Error E05 (Step-by-Step)
Step 1: Total Power Lockdown. Safety first—don’t skip this. Go to your home’s breaker panel and flip the double-pole breaker for the water heater to “Off.” I always tell my guys: use a non-contact voltage tester at the heater to verify the lines are dead before you touch a screwdriver. You’re working with 240V here; it doesn’t give second chances.
Step 2: Access the Lower Thermistor. Use your Phillips head screwdriver to remove the screws holding the lower access panel in place. Pull the panel off and set it aside. You’ll see a thick piece of fiberglass insulation; pull that out carefully (wear gloves if you’re sensitive to it). Behind that, you’ll see the plastic protector over the lower element and the thermistor wires tucked nearby.
Step 3: The Multimeter Test. Disconnect the two wires leading to the thermistor. Set your digital multimeter to the “Ohms” (resistance) setting. Touch the probes to the two terminals of the thermistor. At room temperature (around 77°F), you should see approximately 10,000 ohms (10k). If your meter reads “0” or “0.2,” the sensor is shorted. If it reads “OL” (Open Loop), it’s a different failure, but the fix is the same: replacement.
Step 4: Draining the Tank (Partial). You don’t need to empty the whole thing, but you need the water level to be below the thermistor port. Hook a hose to the drain valve at the bottom, run it to a floor drain, and let it rip for about 5-10 minutes. Warning: The water is hot. Be careful not to scald yourself or damage the hose.
Step 5: Swap the Sensor. Use your socket or wrench to unscrew the old thermistor from the tank. It might be stubborn due to scale buildup. Once out, thread the new AO Smith genuine thermistor in by hand to avoid cross-threading, then snug it up with the wrench. Don’t over-tighten; you don’t want to crack the gasket. Plug the wire harness back in until you hear it click.
Step 6: Refill and Reboot. Open a hot water faucet upstairs to let the air out of the lines. Open the cold water intake valve. Once a steady stream of water flows from the faucet, you know the tank is full. Never turn the power on until the tank is full, or you’ll dry-fire and fry your elements instantly. Flip the breaker back on and check the display. The E05 should be gone.
- Difficulty: Intermediate – Requires basic electrical testing and manual dexterity.
- Estimated Time: 45 to 60 minutes.
- Tools Needed: Phillips head screwdriver, Digital Multimeter, 1-1/16″ socket or a large adjustable wrench, and a few dry towels.
- Estimated Cost: $20 – $45 (depending on your specific model’s replacement part).
Detailed Diagnosis: Root Causes
Why does a thermistor short out? It’s rarely just “bad luck.” In my experience, there are three main culprits behind an E05 error:
- Sediment Encapsulation: This is the big one. Over years, calcium and lime scale build up at the bottom of the tank. This sediment can literally bury the thermistor. Instead of sensing water, it’s sensing “hot mud,” which causes it to overheat and eventually causes the internal resistor to melt and short out.
- Thermal Cycling Fatigue: Every time your water heater kicks on, that sensor expands and contracts. After thousands of cycles, the delicate wiring inside the thermistor probe can fray or touch the metal casing, creating a direct short to ground.
- Moisture Intrusion: If your lower heating element gasket has a microscopic leak, water can seep into the insulation and reach the thermistor terminals. Water and electricity don’t mix; this moisture creates a bridge between the wires, triggering the “shorted” code on the control board.
How to Prevent Error E05
If you don’t want to be back under this heater in two years, you need to change your maintenance habits. First, flush your tank annually. Removing the sediment prevents the thermistor from being buried in corrosive sludge.
Second, check your anode rod. When the anode rod is spent, the tank starts “eating” its own components, including the protective casing on your sensors. Lastly, if you live in an area with extremely hard water, consider a whole-house water softener. It’s the single best way to extend the life of every internal component in your AO Smith heater, from the elements to the thermistors.
Frequently Asked Questions
Can I just bypass the thermistor to get hot water temporarily?
Absolutely not. The thermistor is a critical safety device. If you bypass it, the control board won’t know when the water is boiling, which can lead to a T&P valve blowout or, in extreme cases, a tank explosion. It’s not worth the risk.
Will an E05 error cause my electric bill to go up?
In a way, yes. While the heater isn’t running as often, it is trying to heat the entire tank using only the upper element. This is highly inefficient, as the unit has to cycle much more frequently to maintain even a small amount of usable hot water.
Is the lower thermistor the same as the upper one?
On most AO Smith residential models, the parts are identical in terms of resistance specs, but the wire lead lengths might differ. Always check your specific model number on the rating plate to ensure you’re ordering the correct lead length for the lower port.