A. O. Smith tankless heaters show three digit codes. 111 is ignition failure, 121 is loss of flame, and 991 is imperfect combustion, with 101 as the early warning for 991. The 300s are sensors, 610s the fan, 740s the controller link. Most codes are technician work. Owners can check gas, vents and the DIP switch label.
How are A. O. Smith tankless codes organized?
The codes are three digits, and the first digit tells you the area of the heater that reported the problem. This follows the error code sections of A. O. Smith's maintenance sheets for its 140 series, its 510C series and its 520 series on demand heaters. Not every code appears on every model, and A. O. Smith marks a few as applying only to indoor, direct vent or multi unit installations. Find your model number on the rating plate and check it against the sheet for your series.
- 031 and 101. Setup and warning codes.
- 1xx. Ignition and flame: 111 and 121.
- 3xx. Temperature sensors, called thermistors.
- 4xx. The air fuel ratio rod (391) and the flow sensor (441).
- 5xx and 6xx. Gas valves (510 and 551) and fans (611), plus the external pump (631) on the 520 series.
- 7xx. Circuit board, flame sensing and communication codes.
- 9xx. Exhaust temperature (941) and combustion (991).
What does each code mean?
The labels below are A. O. Smith's own, in short form.
| Code | What A. O. Smith calls it |
|---|---|
| 031 | Incorrect DIP switch setting |
| 101 | Warning for the 991 error code |
| 111 | Ignition failure |
| 121 | Loss of flame |
| 311, 321, 331, 341 | Disconnected or short circuited thermistor (the mix varies by model) |
| 391 | Air fuel ratio rod failure |
| 441 | Flow sensor failure (Easy-Link and multi unit systems only) |
| 510, 551 | Abnormal main gas solenoid valve and gas solenoid valve |
| 611 (621 on the 520) | Fan motor fault (exhaust fan motor on the 520 series) |
| 631 | Abnormal external pump (520 series) |
| 651, 661 | Water control, flow adjustment or bypass valve fault (651 is limited to Easy-Link and multi unit systems on the 510C series) |
| 701 | Computer board fault |
| 711 | Gas solenoid valve drive circuit failure (the 520 series adds a high limit switch trip) |
| 721 | False flame detection |
| 741 | Miscommunication between the heater and the remote controller |
| 751 | Miscommunication between the heater and the built in controller (140 and 510C series) |
| 761 | Miscommunication between parent and child units (Easy-Link systems) |
| 941 | Abnormal exhaust temperature (indoor and direct vent indoor models only) |
| 991 | Imperfect combustion |
Two of the pairs are worth spelling out because they show up most often. Our pages on A. O. Smith codes 111 and 121 and codes 101 and 991 go through the checks in more detail.
### Which codes go away on their own?
A. O. Smith's 140 series and 510C series sheets carry a footnote that helps owners. Codes marked with an asterisk, which include 111, 121, 311, 391, 611, 701, 711, 721 and 991 on those sheets, are cleared when water flow stops in a single unit installation. So if the heater shut down mid shower with one of them, closing the hot water taps and opening one again is a fair first step. It resets the display, not the cause. A code that comes back is a real fault.
Codes without the asterisk, such as 031, 441 and 741, do not carry that note. For 510 and 551, the sheets tell the technician to reset the power supply, and to check the wires and valves if the code returns.
### Where does the code show up?
On these heaters the code can appear in two places. A. O. Smith's sheets say it is shown on the remote controller and on the circuit board inside the cabinet, where the 520 series uses a three digit seven segment display and the 140 and 510C series use a green LED. If your controller shows the code, write it down as displayed. If a technician is on site, the board display gives a second reading. On the communication codes, that second reading matters: A. O. Smith's steps for 741 change depending on whether the code shows on the board, the remote, or both.
### What are the DIP switches for?
Code 031 points at a bank of DIP switches on the circuit board. A. O. Smith's sheet lists the settings they control: the set temperature limit (120 degrees or 140 degrees Fahrenheit on the 140 series), the gas type (natural or propane), the model type (indoor or outdoor), the vent length on indoor models, and the high altitude setting. The altitude table steps through elevation bands, from 0 to 2,000 feet up past 7,500 feet, and the sheet says the fan speed is increased automatically for the higher settings.
None of those is a setting an owner should change. A wrong gas type or vent length setting can feed straight into the combustion codes, which is why the sheets send you to the DIP switches when you see 101 or 991.
Which codes can I check myself?
A few, and only to the extent that they involve nothing you have to open.
Gas first. If you smell gas, do not touch a switch or use a phone inside, and leave. Southwest Gas takes emergency calls at 877-860-6020, and 911 works for any suspected leak.
Then, with no gas smell:
- 111 and 121. The first item on A. O. Smith's list for both is to check the gas supply and inlet gas pressure. What you can do is confirm the gas valve to the heater is open and see whether the stove, dryer or furnace on the same gas line is working. Measuring pressure is technician work.
- 101 and 991. Look outside at the vent terminal for a blockage. A. O. Smith's list also includes vent length, terminal spacing, elevation settings and cleaning grease or lint off the burner and fan, which are for a technician.
- 741 and 751. These are about the link between the heater and its controller. A. O. Smith's steps are to confirm the remote controller is the right model, check the wiring connections and confirm the heater has power. If you can see the display is dark, check the outlet and breaker.
- 031. The DIP switches are set at installation for gas type, model type and vent length. Nothing about a working heater should change them. A technician resets them.
Everything else on the list, meaning the thermistors, the air fuel ratio rod, the gas solenoid valves, the fan motor, the circuit board and the flow parts, is diagnosed with a meter and parts lists. A. O. Smith's sheets give the acceptable voltage and resistance ranges for each, and they are written for someone qualified to work on live gas and electrical parts.
### What is the difference between 101 and 991?
A. O. Smith's sheets describe 101 as a warning for the 991 error, and send the 991 code back to the 101 checklist. They do not publish the trip points, so the sheets do not say when a heater shows one instead of the other. Either way, the checklist is the same, and it is worth acting on the first time you see 101.
When should I call a plumber?
Call for any code you cannot clear by closing the taps, for 101 or 991 that returns, for any ignition or flame code that repeats, and any time you smell gas. Do not open the cabinet and do not bypass a safety device.
Our technicians are licensed and insured, and emergency service runs 24/7. You get a free estimate and upfront pricing before any work starts. If you have a Takagi or another brand, see our pages on Takagi codes 111, 121 and 991, the Rinnai code list and Rheem tankless codes. If your tank heater has a blinking status light instead, see A. O. Smith blinking light codes.