AO Smith Tankless Water Heater Troubleshooting
The Short Answer: An AO Smith tankless with no hot water has one of five problems: no 120-volt power, no flow call, a lockout code, failed ignition, or a restriction in water, gas, venting, or condensate. Photograph the display and In Use light before resetting. A repeated gas, vent, or combustion fault needs a licensed technician.
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Search Intent & The Real Problem
AO Smith tankless models share a symptom but not every control sequence. Some are condensing, some are non-condensing, and some are sold under closely related brands. The model plate decides which error chart applies. A blank display is a power problem. A display that is on but never shows a call is a flow or controller problem. An active call with no flame is an ignition or fuel problem.
This distinction saves a lot of unnecessary flushing. Descaling a unit that has no power or no flow signal cannot make the burner start. A fixture that is lukewarm after another fixture runs may have a mixing-valve crossover; it is not automatically a bad AO Smith heat exchanger.
What other guides get wrong
Generic articles tell every AO Smith owner to press reset, open a hot tap, or pour vinegar through the heater. Those steps do not apply to a dark display, a closed isolation valve, a frozen condensate drain, or a flame lockout. They also mix electric tank elements with gas tankless parts. The code and the model plate come first.
Step-by-Step Troubleshooting
Step 1: Identify the exact unit and symptom
| What you see | First branch | What it rules out |
|---|---|---|
| Blank display | Power, disconnect, GFCI, or PCB | Do not start with gas |
| Display on, no In Use or flame | Flow, valve, filter, or control | Burner may be fine |
| In Use on, code appears | Lockout chart | Photograph before reset |
| Flame on, water lukewarm | Flow, scale, gas, mixing, or crossover | Not a dead controller |
| Water or condensate in cabinet | Leak or drain path | Stop firing if board is wet |
- What to look for: Model and serial on the side label, fuel, display code, closest hot fixture, and whether every fixture is cold.
- Required tools: Flashlight, phone, and a contact thermometer if available.
Step 2: Prove the safe power and water basics
- What to look for: Disconnect on, breaker on, outdoor GFCI not tripped, cold and hot isolation valves fully open, and the inlet screen not packed with debris. Reset a GFCI once. If it trips again, leave it off. Open a tub spout instead of a low-flow shower mixer to make a clean call test.
- Required tools: Eyes, a bucket, and a thermometer. Do not open the combustion cover.
If the display remains dead after a known-good receptacle or disconnect, stop. A live board with no screen can be a control-power fault. Do not probe a live gas appliance unless you are trained and equipped for it.
Step 3: Check whether the heater sees flow
- What to look for: In Use indicator, water moving at the fixture, isolation handles parallel to the pipe, and a minimum-flow problem at a restrictive aerator. If the indicator never comes on, remove a clogged faucet aerator or test at the tub spout. A recirculation crossover can also make temperatures look wrong.
- Required tools: Bucket, flashlight, and contact thermometer.
The inlet screen may be cleaned only with the unit isolated according to the manual. Do not screwdriver a live flow sensor or servo. If a tub spout creates a call but the shower does not, the heater is responding and the fixture is the next suspect.
Step 4: Read the code instead of resetting blindly
- What to look for: Exact digits, letters, or flashing pattern. AO Smith code meanings vary by platform. A combustion or ignition code that returns after one power cycle is not a reset problem.
- Required tools: Phone photo and the model-specific manual.
Typical residential NG supply may be around 3.5–7 inches water column and LP around 10–11, but the rating plate controls. Gas pressure, combustion readings, vent proving, and valve testing are licensed work. Never purge a gas line or loosen a fitting.
Step 5: Separate lukewarm from no heat
- What to look for: Heater outlet temperature compared with the nearest faucet. Hot at the unit but cold at the tap points to a mixing valve or crossover. Hot briefly then cold points to flow loss, scale, gas supply, or an overheating shutdown. A condensate trap full of water can lock out a condensing model.
- Required tools: Contact thermometer and a dry flashlight inspection.
Scale service is reasonable only when the unit is designed for isolation flushing and the manual calls for it. It is not a cure for a failed ignition, wet board, blocked vent, or split exchanger.
When DIY Ends: Calling a Licensed Pro
Call when a code returns, the blower is silent, the flame fails, the gas control is involved, the cabinet is wet, the breaker or GFCI trips, or the unit still cannot make hot water after the valve and flow checks. Give the technician the model, code, timing, and photos. That is more useful than ten resets and a cabinet full of replacement parts.
Frequently Asked Questions
Q: Why is my AO Smith tankless water heater not heating? A: Start with power, open isolation valves, flow at a tub spout, and the exact display code. If the unit calls but will not ignite, or a code returns, gas, vent, and combustion diagnosis belongs to a licensed technician.
Q: Should I reset my AO Smith tankless water heater? A: You can power-cycle once after correcting an obvious tripped GFCI or closed valve. Do not repeatedly reset ignition, flame, exhaust, or overheat codes. The repeated fault is the diagnosis clue.
Q: Can low water flow cause an AO Smith tankless to go cold? A: Yes. A clogged inlet screen, restrictive fixture, closed isolation valve, or crossover can drop flow below activation. Test with a tub spout. If the In Use light still will not come on, call for a flow or control diagnosis.