What do A. O. Smith tankless codes 101 and 991 mean?

A. O. Smith's maintenance sheets list 991 as imperfect combustion and 101 as a warning for the 991 error code. The sheet for 991 in every series sends you to the 101 checklist, so the two are one problem at two stages. A. O. Smith does not publish the trip points in these sheets, so treat 101 as the early notice and 991 as the fault.

Two facts about how the heater behaves:

• 101 is labeled a warning, not a fault, so it is the one to act on first.
• 991 can clear itself. On the 140 series and 510C series sheets, 991 carries the asterisk footnote that says the code is cleared when water flow stops in a single unit installation. That resets the display. If the cause is still there, the code comes back.

The 101 checklist is the same idea across the three sheets, with small changes for each model. In A. O. Smith's words, in short form:

1. 1The gas type. The 140 series sheet says to check the gas type of the house or building. The 510C series sheet adds that the heater comes from the factory set for natural gas, and can be converted to propane by a qualified agent with the LP conversion kit that comes with it.
2. 2Blockage in the intake air or exhaust. The 520 series sheet lists examples: a stuck damper, vent flaps installed on the terminator, snow built up around the terminator, and a heater installed in a closet with no ventilation or combustion air. The 510C sheet says to remove any blockage in the concentric venting system.
3. 3Terminal spacing. For a direct vent system, is there enough distance between the intake air terminal and the exhaust terminal? The 510C sheet asks the same about the distance between a concentric terminal and other exhaust terminals.
4. 4Vent length. The 520 sheet says the total vent length should not exceed 50 feet, with fewer than five elbows. The 510C sheet says to verify the length is within the maximum and that the DIP switches are set for it.
5. 5Elevation. Check the elevation where the heater is installed and the matching high altitude DIP switch setting.
6. 6Grease or dirt in the burner and the fan motor, especially if the heater is installed in a contaminated area.
7. 7Dust and lint in the heat exchanger.
8. 8Manifold pressure, from the rating plate or the installation manual.

### How do the three series differ?

The checklists overlap, and each sheet has details the others lack.

• 140 series. Asks about the gas type of the house or building, blockage in the intake air and exhaust, terminal spacing for direct vent installs, total vent length, the altitude DIP switches, grease or dirt in the burner and fan, lint in the heat exchanger and manifold pressure. It has separate DIP switch positions for indoor and outdoor models.
• 510C series 110. Written for a concentric vent, where intake and exhaust share one terminal. It adds the factory natural gas setting and the propane conversion kit, and it ties the vent length limit to the DIP switch setting.
• 520 series. The most specific about the installation. It says that if the heater is the wrong gas type model, the fix is to replace it with the correct one. It gives the 50 foot, fewer than five elbows limit, and it lists the stuck damper, added vent flaps, snow and closet installs.

Read down any of the three lists and most items are about how the heater was installed and what surrounds it, with fewer about what has built up inside.

That is a lot of items, and most of them mean opening the cabinet, measuring gas pressure or working from the installation manual. This is technician work on a gas appliance.

One thing, plus a look at how the room is used.

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, walk outside and look at the vent terminal. A. O. Smith's examples of blockage include a stuck damper, vent flaps installed on the terminator and snow built up around it. Arizona homes rarely see snow, but a terminal covered by something stored or built against it counts the same way. If something plainly blocks a terminal you can reach from the ground, take it away with the heater off. If the terminal is on the roof or behind a wall, leave it for the technician.

Look at the room. The 520 sheet calls out a heater installed in a closet with no ventilation. If yours sits in a closet, garage corner or utility room where paint, aerosols, cleaning products or storage are stacked close, ask the technician about it. Our page on water heater clearance and access code covers the space a gas water heater needs, and our page on the 18 inch rule in a garage covers flammable vapors.

### Should I worry about carbon monoxide?

The CDC recommends having your water heater and other gas burning appliances serviced by a qualified technician every year, and installing battery operated or battery backup carbon monoxide detectors near every sleeping area. It also says to check the detector batteries when you change your clocks in spring and fall. A combustion code is a good prompt to test yours.

None of that is a reason to panic over a 101. It is a reason to book the service call promptly and to confirm your detectors work before you do.

### What should I do while I wait for service?

If 991 has locked the heater out, leave it off. Do not keep cycling the power to clear the code, and do not try to bypass the combustion sensing. Note the code, the time, what the heater was doing and whether any other codes appeared. Keep the model number from the rating plate, because A. O. Smith's checklist changes between series.

Call as soon as 101 appears twice, or the first time you see 991. A repeated 101 means the warning is not going away. Also call if you find a blocked vent you cannot safely clear, if the heater sits in a closet with no combustion air, or if you smell gas at any point.

HQ Plumbing and Air answers emergency calls around the clock, and the crew is licensed and insured. Estimates cost nothing, and pricing is given upfront. For the rest of the code list, see our page on A. O. Smith tankless error codes and our page on A. O. Smith codes 111 and 121. Takagi builds a similar three digit code set, and our page on Takagi codes 111, 121 and 991 compares.