AO Smith Tankless With No Hot Water
The Short Answer: An AO Smith ATI, 240, 340, or 540 tankless with no hot water is no 120 V, no flow call, a lockout (10/11/12/14/16), or gas that never opened. Photograph the remote. Do not vinegar-flush a dark controller. One power cycle after you fix the cause.
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Search Intent & The Real Problem
Every fixture is cold. Not slow. Not lukewarm. Cold. Most residential AO Smith tankless units are Takagi platforms in an AO Smith jacket — ATI non-condensing, 240/340/540 condensing. Forums paste Rinnai 11 onto a unit that is actually a 10 because a bird screen packed the concentric vent. This page is no heat at all. Cousin chassis: Takagi problems. Reset procedure: AO Smith tankless reset. A glass-lined ProMax puddle is AO Smith from the bottom, not this wall-hung box.
What other guides get wrong
They start with a vinegar flush. Flushing a unit that never fires floods a mixer and does nothing. They treat every AO Smith tankless like a ProLine tank with a red ECO. There is often no customer high-limit button on the jacket. They skip the rating plate, so a 540P condensate trap gets a “clean the burner” video meant for an outdoor ATI. And they tell you to crank the remote to 140°F. A dead exchanger does not care about setpoint.
Step-by-Step Troubleshooting
Step 1: Display, flow, and the exact cold
| What you have | Folder | Next |
|---|---|---|
| Blank remote / dark PCB LED | Power / GFCI / PCB | Step 2 |
| Display On, no fire, no code | No flow call | Inlet screen, isolation valves, min GPM |
| Two-digit code on remote or LED | Lockout | Photograph; Step 4 |
| Fan runs, no flame, 11/12 | Ignition / gas | Cock, then licensed |
| Flame on, water still cold | Mixing valve / crossover | Thermometer at the outlet |
| After a storm or freeze | Outage / rain / split | After outage, after rain |
- What to look for: Closest hot faucet as the test, not the master shower 60 feet away. Write the model (ATI-110, ATI-510, 540P, and cousins) off the rating plate — parts and codes move between generations.
- Required tools: Phone photo of the remote before you cycle power.
Lukewarm-then-hot is pipe lag, not this article. Kitchen-only cold with baths fine is often a fixture, not the heater — kitchen sink path.
Step 2: 120 V — GFCI, disconnect, the remote cable
- What to look for: Outdoor GFCI popped. Disconnect off. House breaker off. Remote unplugged so the pad is dead while the PCB still has power. Reset a GFCI once. If it trips again, leave it off — breaker will not reset and tankless trips breaker are shorts, not a code.
- Required tools: Eyes, receptacle tester. Do not hold a tripped breaker On.
A 540 that died after a garage freeze with power off is often a split exchanger, not a GFCI.
Step 3: Call for heat — valves, screen, activation flow
- What to look for: Isolation handles not parallel to the pipe. Cold-inlet screen packed with grit or Teflon scraps from a new install. Fixture mixing below ~0.4–0.75 GPM on these chassis. Recirc crossover that cancels net flow. Open a tub spout. If the unit never wakes, stay on flow — Noritz no-flow physics — do not start on gas.
- Required tools: Filter service, bucket. Isolation off before the screen comes out.
No service valves is an incomplete install. Do not unscrew the house shutoff and hope.
Step 4: Read the code — then one reset, not ten
| Code (ATI / 240 / 340 / 540 family) | Meaning | First check |
|---|---|---|
| 10 | Air / exhaust | Termination, bird screen, condensate on 540 |
| 11 | No ignition | Gas cock open, igniter, lockout after failed trials |
| 12 | Flame lost | Flame rod, low gas, wind |
| 13 | Remaining flame | Shut gas — licensed |
| 14 / 16 | Overheat / outlet too hot | Scale, low flow, dry fire |
| 31 / 32 | Outlet / inlet thermistor | Unplugged sensor, scale on probe |
| 61 / 71 / 72 | Fan / solenoid / board | Licensed |
| None, just dead | Power or flow | Steps 2–3 |
- What to look for: One power cycle after the obvious cause (cap, hose, GFCI, cock parallel). Same digit on the next shower is the end of DIY. Reset how-to: AO Smith tankless reset. Takagi twins use the same digits — Takagi reset.
- Required tools: Jacket sticker. No manometer. NG ~3.5–7 in. w.c. / LP ~10–11 in. w.c. is licensed — confirm the plate. Do not purge the line.
A 13 is not DIY. Fuel may still be flowing after the burner should be dead.
Step 5: Flame on, water still cold — not the burner
- What to look for: 120°F at the AO Smith outlet, 100°F at the fixture = mixing valve or crossover. Dip-tube thinking does not apply to tankless. Gas on, fan on, water never rises: scale-insulated exchanger — licensed flush or parts. House static over ~80 psi lifting a T&P is pressure, not “no hot water,” but it will dump the heat you paid for — Rinnai T&P.
- Required tools: Contact thermometer at the outlet vs the faucet.
Do not set 140°F to chase a mixer.
When DIY Ends: Calling a Licensed Pro
You may restore one GFCI, rinse the inlet screen, open isolation valves, photograph a code, and power-cycle once. Stop at gas odor, a wet cabinet, a silent fan, a freeze-split, a 13, a repeating 10/11/12, and any jumper. Combustion, gas pressure, flame rod, PCB, and exchanger work are licensed. A packed screen is a rinse. A 12 that returns is a service truck. AO Smith will not love a 540 exchanger that was reset through overheat all weekend.
Frequently Asked Questions
Q: Why does my AO Smith tankless have no hot water? A: No 120 V, flow below activation, a packed inlet screen, a lockout code (10/11/12/14/16), or a mixing valve hiding a hot outlet. Photograph the remote before you cycle power.
Q: How do I reset an AO Smith tankless with no hot water? A: Photograph the code, fix the obvious cause, power off 10–30 seconds. One try. A code that returns is licensed. Do not reset a wet cabinet or a unit that smells like gas.
Q: Should I flush an AO Smith tankless that will not heat at all? A: Not first. Flushing treats scale on a unit that fires. A dark controller, a 10, or a 13 needs power, vent, or a shut-gas call — not acid.