What do A. O. Smith tankless codes 111 and 121 mean?

A. O. Smith's maintenance sheets label 111 as ignition failure and 121 as loss of flame.

• 111. The heater tried to light and did not. There was no flame to begin with.
• 121. The heater lit, and then the flame went out or could not be confirmed.

Think of it as before and after. With 111, the problem is in the start: gas arriving, a spark, the gas valves opening. With 121, the start worked and something interrupted the burn. The sheet's list of suspects is gas supply, a sooty flame rod, a heat exchanger leak and lint in the manifold nozzles.

On the 140 series and 510C series sheets, both codes carry the asterisk footnote that says they clear when water flow stops in a single unit installation. That resets the display when you close the taps. It does not fix what caused the code.

Both codes open with the same two items. Everything after that reads like a technician's route through the heater, from the outside in.

### The shared checks

1. 1Gas supply and inlet gas pressure. This is the first line for both codes on all three sheets. A meter reading at the heater's inlet tells the technician whether the gas system is delivering enough.
2. 2The high limit switch. Is it working properly?
3. 3Wires and the flame rod. The sheets ask for a check of the wire connections for breaks, burn marks on the circuit board and soot on the flame rod. If the part the sheet calls the O.H.C.F. is broken, the sheet says to consult the manufacturer.
4. 4Heat exchanger leaks. Is there any leaking from the heat exchanger?
5. 5Dust and lint in the manifold nozzles. The gas nozzles can clog.
6. 6Flame rod current. A. O. Smith's appendix gives the normal reading as more than 5 microamps when there is a flame on the 140 and 510C series, and more than 1 microamp on the 520 series. If the current is normal with a flame, the sheet says to replace the circuit board. If it is not, replace the flame rod.

### The extra steps for 111

Because 111 means the burner never lit, the sheets add a listening test. During the ignition stage the technician is told to check for two sounds:

• A buzzing spark ignition sound from the burner as the heater prepares for combustion.
• A double clunk from the gas valve assembly as the heater goes into combustion.

The reading depends on which sound is missing. With no spark, the sheet sends the technician to the igniter voltage check, where the normal reading on the 140 and 510C series is 108 to 132 volts AC across the purple wires (the 520 series sheet gives 90 to 110 volts AC), and to replace the igniter assembly if the voltage is normal. With no clunk, the sheet sends the technician to voltage and resistance checks at the gas valve assembly. Those checks are done with a meter on live parts.

Less than you might hope, and that is by design. The list above is for a qualified person. There are three owner steps that help.

Gas smell first. If you smell gas, do not light or switch anything, do not use a phone inside, and leave. Southwest Gas takes emergency calls at 877-860-6020, and 911 works for any suspected leak.

With no smell:

1. 1Confirm the gas to the heater is on. The valve on the gas line at the heater should be open. If other gas appliances in the house are working, gas is reaching the home.
2. 2Clear the code once. Close the hot water taps so flow stops, then open one hot tap and see whether the heater starts. If it lights and runs, note the date. If the code returns, it is not a one time event.
3. 3Listen, and write it down. When a hot tap opens and the heater tries to start, listen from a safe distance. Can you hear a buzzing spark, and then a double clunk from the gas valves? Tell the technician which of those you heard and which was missing. A. O. Smith's own checklist depends on it.

Do not remove the front cover, clean the flame rod or bend wires. The flame rod is a sensor whose position and cleanliness matter to combustion, and the sheets list soot on it among the technician's checks. Do not bypass the high limit switch or any safety device.

### What if the heater will not light and shows no code?

The 520 series sheet covers this too. For a unit that does not ignite when water goes through it, A. O. Smith lists three checks. If you use the remote controller, turn its power button on, and the green LED will light. Check that the filter on the cold water inlet is clean. And look at the power supply and water circuit. The inlet filter is a common miss, since a clogged one starves the heater of flow. Our page on common tankless water heater problems covers the flow related causes.

For a heater that runs but gives water that is not hot enough or turns cold and stays cold, the sheet points at cross plumbing between the hot and cold lines, a gas valve that is not fully open, a gas line that is undersized, and the set temperature and DIP switch setting. Two are owner checks: confirm the gas valve at the heater is fully open, and confirm the temperature setting on the controller. Cross plumbing and gas line sizing are for a plumber.

### Which related codes should I know?

Three neighbors point back to the same parts:

• 711, gas solenoid valve drive circuit failure. On the 140 series sheet, A. O. Smith's action is to refer to the 111 and 121 codes. On the 520 series sheet, 711 is listed as a high limit switch trip or gas solenoid valve drive circuit failure.
• 721, false flame detection. The heater sees a flame when there should not be one. The sheet says to clean the flame rod and check for a leak in the heat exchanger. The 520 series sheet adds that for indoor models you should check that a condensate drain is installed on the vent collar.
• 510 and 551, abnormal main gas solenoid valve and gas solenoid valve. The sheets say to reset the power supply and check the wires, then check the voltage on each valve in the gas valve assembly.

Call when 111 or 121 returns after you have closed the taps and tried once more, when the heater will not start at all, when you hear the spark but no gas valve clunk, or any time you smell gas. A tankless that keeps locking out has a fault, and repeated attempts to light it only stress the parts.

For a repeating ignition or flame code, HQ Plumbing and Air is available 24/7, licensed and insured, and gives free estimates with upfront pricing. For every A. O. Smith code, see our page on A. O. Smith tankless error codes, and for the combustion warning, see A. O. Smith codes 101 and 991. If your heater is a Rinnai, our pages on Rinnai error code 11 and Rinnai error code 12 cover the same two moments under different numbers.