A. O. Smith Tankless No Hot Water: Checks and Codes

The Short Answer: An A. O. Smith gas tankless that gives no hot water usually has one of five problems: no 120-volt power, too little water flow to start the burner, a closed gas or water valve, a clogged inlet filter, or a fault code. Read the controller or the board’s light before resetting anything. The code meaning depends on the model series, so match it to your manual.

Which A. O. Smith tankless do you have?

A. O. Smith has sold several generations of gas tankless heaters, and they do not all show faults the same way. Find the model on the rating plate on the side panel:

• 240, 340 and 540 condensing models (for example the ATI-540H). Manual 2000534710 (opens in a new tab) covers the Indoor and Outdoor versions and shows three-digit codes on the controller.
• Older 110, 310 and 510 non-condensing models, also labeled T-KJr2, T-K4 and T-D2. On their manual (opens in a new tab), the 510 shows a three-digit code on the circuit board. The 110 and 310 blink a red LED on the board, or show a code on a remote controller if one is installed.
• Other current models (for example the X3 versions, the ATI-540P, electric tankless units, and models sold under other A. O. Smith family brands) have their own manuals. Look yours up in A. O. Smith’s manual lookup (opens in a new tab) before using the table below.

What you can check safely

These are the checks A. O. Smith’s owner’s guide lists for “hot water is not available when a fixture is opened”. Each one is safe for an owner.

1. Power. The heater needs 120-volt power to ignite, even on gas. Check the plug, the outlet or GFCI, and the breaker. Reset a tripped breaker or GFCI once. If it trips again, leave it off. A blank controller means no power, or a controller that has gone to sleep. The controller display turns off after five minutes without use.
2. Controller power button. If a remote or temperature controller is installed, make sure it is switched on.
3. Gas and water valves. The gas shutoff and the water isolation valves at the heater should be fully open. On propane, check that the tank is not empty.
4. Flow. The unit needs at least 0.5 gallons per minute through it to start. Open a tub spout fully. If the tub gets hot but a shower or faucet does not, the fixture, its aerator or a mixing valve is the restriction. See tankless water heater no water flow for flow problems.
5. Inlet filter. A clogged cold-water inlet filter cuts flow. The owner’s guide lists cleaning it as routine maintenance, done only after shutting off electrical power, gas and water as the manual describes. If you are not comfortable doing that, include it in a service visit.
6. Freezing. Freeze protection only works while the unit has power. It switches on below 36.5°F. A unit that lost power in freezing weather may be frozen, and freeze damage from a power loss is not covered by the warranty.
7. Inlet temperature. If the incoming water is already close to the set temperature, for example from solar preheating, the heater will not fire.

Hot, then cold, or never hot enough

• Goes cold and stays cold: flow too low to keep the burner running, a partly closed gas valve, a clogged filter, or a recirculation loop missing check valves. The guide lists all four.
• Not hot enough: the set temperature is low, the flow is more than the unit can heat with your incoming water, or a cold-to-hot crossover (such as a faulty mixing valve) is blending cold water in. Gas line size and gas pressure are also on the list. Those checks belong to a licensed technician. For the general pattern, see tankless goes from hot to cold.
• Fan keeps running after the tap closes: normal. The manual says the fan runs for 15 to 70 seconds after operation to clear the flue.

Hard water also matters. The manual for the 240, 340 and 540 models limits hardness to 7 grains per gallon (120 ppm) in single-family homes. Scale from harder water can damage the unit, and the warranty does not cover damage caused by water quality.

Error codes on the 240, 340 and 540 models

These meanings come from manual 2000534710 (opens in a new tab). They apply to the 240, 340 and 540 Indoor and Outdoor models only.

Codes 441, 651 and 761 relate only to linked multi-unit (Easy-Link) systems. In a linked system the display also shows which unit has the fault.

The older 110, 310 and 510 models use many of the same three-digit numbers, but not all of them. For example, 331 is a mixing thermistor failure on the older models. Use the table in the manual for your series.

What the technician will check

For ignition, flame and combustion codes (111, 121, 101, 991, 721), the technician checks the gas supply pressure at the heater, with the burner both idle and at full fire. They confirm the gas type matches the unit and inspect the vent and air intake for blockage or recirculation. They also look at the burner, fan and flame rod for dirt or soot, and check the condensate drain on condensing models. Gas pressure testing and cabinet work are licensed tasks, and A. O. Smith’s manual says any repairs should be done by a licensed technician. For a code that clears after one power cycle and then returns, the code itself is the evidence to show the technician. The A. O. Smith tankless reset guide covers when a reset is reasonable.

Frequently Asked Questions

Q: My A. O. Smith tankless shows 111 after a power outage. Is it broken? A: Code 111 means ignition failure. After an outage, first confirm that the gas valve is open and other gas appliances work. On propane, check the tank level. If 111 comes back after a single reset, a technician needs to check the gas supply and ignition parts.

Q: The screen is blank. Is the heater dead? A: Not necessarily. The controller display turns off after five minutes without use. Open a hot tap. If the display stays blank with water flowing, check the outlet, GFCI and breaker. A heater with confirmed power and a blank display needs service.

Q: Why does only one faucet run cold? A: If other fixtures get hot, the heater is working. A low-flow aerator, a showerhead restrictor, or a faulty mixing valve on that fixture is the more likely cause. The heater needs at least 0.5 GPM to start.