AO Smith Water Heater Error Codes

Look up what a blink or error code means on your AO Smith water heater, and whether it is safe to check yourself.

This is a gas appliance. If you smell gas or hear hissing, leave the house and call your gas utility or 911 from outside. Repairs to gas lines or burners are a job for a licensed technician.

This table covers 7 verified AO Smith gas control families, both tankless and tank models. AO Smith's condensing tank lines, such as Vertex and ProLine, are not covered here.

Search for your code

AO Smith tankless (on demand) gas water heaters, model 140 family (Indoor and Outdoor), 3 digit numeric codes on the built in or remote control digital display

Code	Severity	What it means	Safe checks
031	Pro required	The control board's switches are not set the way the water heater needs, so it will not run correctly.	None needed
101	Safety hazard	The water heater is warning that a vent blockage, wrong gas type, altitude setting, or dirty burner could soon cause a shutdown for bad combustion.	2 safe checks Look at the outside vent pipe and the fresh air intake pipe for blockage such as snow, ice, leaves, or an animal nest Look at other gas appliances in the home, such as the stove or furnace, to see if they are working. This tells you if the home's gas supply is on
111	Safety hazard	The water heater tried three times to light the burner and failed, so it shut off the gas for safety.	1 safe check Look at other gas appliances in the home to see if they are working. This tells you if the home's gas supply is on
121	Safety hazard	The burner flame went out unexpectedly while the water heater was running.	1 safe check Look at other gas appliances in the home to see if they are working. This tells you if the home's gas supply is on
311, 321, 341	Pro required	A water temperature sensor inside the unit has a broken wire or has failed, so the control cannot read the temperature correctly.	None needed
391	Safety hazard	A sensor rod that helps balance the mix of gas and air during burning is not working correctly.	None needed
510, 551	Safety hazard	The valve that controls gas flow to the burner is not opening or closing the way it should.	None needed
611	Safety hazard	The fan that moves combustion air and exhaust is not working correctly.	1 safe check Look at the outside vent pipe for blockage such as snow, ice, leaves, or an animal nest
701	Pro required	The main control board inside the water heater has failed.	None needed
711	Safety hazard	The electrical circuit that operates the gas valve has failed.	None needed
721	Safety hazard	The control thinks it senses a flame even though the burner is supposed to be off.	None needed
741	Pro required	The water heater and its plug in remote control panel are not communicating with each other correctly.	1 safe check Look at the home's breaker panel to see if the breaker for the water heater has tripped
751	Pro required	The water heater and its built in control panel are not communicating with each other correctly.	1 safe check Look at the home's breaker panel to see if the breaker for the water heater has tripped
941	Safety hazard	The exhaust gas leaving the unit is hotter than it should be. This code only appears on the 140 Indoor model.	1 safe check Look at the outside vent pipe for blockage such as snow, ice, leaves, or an animal nest
991	Safety hazard	The burner is not burning gas cleanly and completely, most often because of a blocked vent or not enough combustion air.	2 safe checks Look at the outside vent pipe and fresh air intake pipe for blockage Look at other gas appliances in the home to see if they are working

Sources

AO Smith Maintenance Sheet 88A166-3, model 140 tankless water heater, error codes and wiring diagram (accessed 2026-07-03)

AO Smith tankless (on demand) gas water heaters, model 520 family (Direct Vent Indoor, Outdoor, and Easy-Link parent/child systems), 3 digit numeric codes on the built in or remote control digital display

Code	Severity	What it means	Safe checks
031	Pro required	The control board's switches are not set the way the water heater needs, so it will not run correctly.	None needed
101	Safety hazard	The water heater is warning that a vent blockage, wrong gas type, altitude setting, or dirty burner could soon cause a shutdown for bad combustion.	2 safe checks Look at the outside vent pipe and the fresh air intake pipe for blockage such as snow, ice, leaves, or an animal nest Look at other gas appliances in the home to see if they are working. This tells you if the home's gas supply is on
111	Safety hazard	The water heater tried three times to light the burner and failed, so it shut off the gas for safety.	1 safe check Look at other gas appliances in the home to see if they are working. This tells you if the home's gas supply is on
121	Safety hazard	The burner flame went out unexpectedly while the water heater was running.	1 safe check Look at other gas appliances in the home to see if they are working. This tells you if the home's gas supply is on
311, 321, 331, 341	Pro required	A water temperature sensor inside the unit has a broken wire or has failed, so the control cannot read the temperature correctly.	None needed
391	Safety hazard	A sensor rod that helps balance the mix of gas and air during burning is not working correctly.	None needed
441	Pro required	The water flow sensor, which is only used on Easy-Link linked systems, is not measuring water flow correctly.	None needed
510, 551	Safety hazard	The valve that controls gas flow to the burner is not opening or closing the way it should.	None needed
611, 621	Safety hazard	One of the fans that moves combustion air or exhaust is not working correctly.	1 safe check Look at the outside vent pipe for blockage such as snow, ice, leaves, or an animal nest
631	Pro required	An external water pump connected to the system is not working correctly.	1 safe check Look at the home's breaker panel to see if the breaker for the pump has tripped
651, 661	Pro required	A motorized valve that controls water flow, used only on Easy-Link systems, is stuck or not working, often because of mineral buildup or a water leak.	None needed
701	Pro required	The main control board inside the water heater has failed.	None needed
711	Safety hazard	Either the high temperature safety switch has tripped, or the electrical circuit that operates the gas valve has failed.	None needed
721	Safety hazard	The control thinks it senses a flame even though the burner is supposed to be off.	None needed
741	Pro required	The water heater and its plug in remote control panel are not communicating with each other correctly.	1 safe check Look at the home's breaker panel to see if the breaker for the water heater has tripped
761	Pro required	In a system with more than one linked water heater (Easy-Link), the main unit and a connected unit are not communicating correctly.	1 safe check Look at the home's breaker panel to see if the breaker for the water heater has tripped
941	Safety hazard	The exhaust gas leaving the unit is hotter than it should be. This code only appears on the 520 Direct Vent Indoor model.	1 safe check Look at the outside vent pipe for blockage such as snow, ice, leaves, or an animal nest
991	Safety hazard	The burner is not burning gas cleanly and completely, most often because of a blocked vent or not enough combustion air.	2 safe checks Look at the outside vent pipe and fresh air intake pipe for blockage Look at other gas appliances in the home to see if they are working

Sources

AO Smith Maintenance Sheet 60E101, model 520 tankless water heater, error codes and wiring diagram (accessed 2026-07-03)

AO Smith tankless (on demand) gas water heaters, model 110 family (Direct Vent Indoor and Outdoor). These units are read two ways: a 2 digit numeric code on a plug in remote display, or a single red LED on the control board that blinks a set number of times. Code field below lists the numeric code plus the documented blink count for the red LED

Code	Severity	What it means	Safe checks
03 (red LED blinks 1 time)	Pro required	The control board's switches are not set the way the water heater needs, so it will not run correctly.	None needed
10 (red LED blinks 5 times)	Safety hazard	The water heater is warning that a vent blockage, wrong gas type, altitude setting, or dirty burner could soon cause a shutdown for bad combustion.	2 safe checks Look at the outside vent pipe and the fresh air intake pipe for blockage such as snow, ice, leaves, or an animal nest Look at other gas appliances in the home to see if they are working. This tells you if the home's gas supply is on
11 (red LED blinks 3 times)	Safety hazard	The water heater tried three times to light the burner and failed, so it shut off the gas for safety.	1 safe check Look at other gas appliances in the home to see if they are working. This tells you if the home's gas supply is on
12 (red LED blinks 3 times)	Safety hazard	The burner flame went out unexpectedly while the water heater was running.	1 safe check Look at other gas appliances in the home to see if they are working. This tells you if the home's gas supply is on
31, 32 (red LED blinks 2 times)	Pro required	A water temperature sensor inside the unit has a broken wire or has failed, so the control cannot read the temperature correctly.	None needed
39 (red LED blinks 2 times)	Safety hazard	A sensor rod that helps balance the mix of gas and air during burning is not working correctly.	None needed
51, 55 (red LED blinks 6 times)	Safety hazard	The valve that controls gas flow to the burner is not opening or closing the way it should.	None needed
61 (red LED blinks 4 times)	Safety hazard	The fan that moves combustion air and exhaust is not working correctly.	1 safe check Look at the outside vent pipe for blockage such as snow, ice, leaves, or an animal nest
70 (red LED blinks 1 time)	Pro required	The main control board inside the water heater has failed.	None needed
72 (red LED blinks 6 times)	Safety hazard	The control thinks it senses a flame even though the burner is supposed to be off.	None needed
74 (blink count not given in source document)	Pro required	The water heater and its plug in remote control panel are not communicating with each other correctly.	1 safe check Look at the home's breaker panel to see if the breaker for the water heater has tripped
99 (red LED blinks 5 times)	Safety hazard	The burner is not burning gas cleanly and completely, most often because of a blocked vent or not enough combustion air.	2 safe checks Look at the outside vent pipe and fresh air intake pipe for blockage Look at other gas appliances in the home to see if they are working

Sources

AO Smith Maintenance Sheet 62Q064 (ATKJ2), model 110 tankless water heater, error codes, red LED blink counts, and wiring diagram (accessed 2026-07-03)

AO Smith tankless (on demand) gas water heaters, model 540P family (Indoor, Outdoor, and Easy-Link systems with a recirculation pump), 3 digit numeric codes on the built in or remote control digital display

Code	Severity	What it means	Safe checks
031	Pro required	The control board's switches are not set the way the water heater needs, so it will not run correctly.	None needed
101	Safety hazard	The water heater is warning that a vent blockage, wrong gas type, altitude setting, or dirty burner could soon cause a shutdown for bad combustion.	2 safe checks Look at the outside vent pipe and the fresh air intake pipe for blockage such as snow, ice, leaves, or an animal nest Look at other gas appliances in the home to see if they are working. This tells you if the home's gas supply is on
111	Safety hazard	The water heater tried three times to light the burner and failed, so it shut off the gas for safety.	1 safe check Look at other gas appliances in the home to see if they are working. This tells you if the home's gas supply is on
121	Safety hazard	The burner flame went out unexpectedly while the water heater was running.	1 safe check Look at other gas appliances in the home to see if they are working. This tells you if the home's gas supply is on
311, 321, 331, 341, 351	Pro required	A water temperature sensor inside the unit has a broken wire or has failed, so the control cannot read the temperature correctly.	None needed
391	Safety hazard	A sensor rod that helps balance the mix of gas and air during burning is not working correctly.	None needed
441	Pro required	The water flow sensor is not measuring water flow correctly. In cold weather this can also happen if water in the unit's pump piping has frozen.	None needed
510, 551	Safety hazard	The valve that controls gas flow to the burner is not opening or closing the way it should.	None needed
611	Safety hazard	The fan that moves combustion air and exhaust is not working correctly.	1 safe check Look at the outside vent pipe for blockage such as snow, ice, leaves, or an animal nest
631	Pro required	The built in recirculation pump is not working, or water inside the pump has frozen.	1 safe check Look at the home's breaker panel to see if the breaker for the water heater has tripped
651	Pro required	A motorized flow adjustment valve, used only on Easy-Link systems, is stuck or not working, often because of mineral buildup or a water leak.	None needed
661	Pro required	A motorized bypass valve is stuck or not working, often because of mineral buildup or a water leak.	None needed
701	Pro required	The main control board inside the water heater has failed.	None needed
711	Safety hazard	The electrical circuit that operates the gas valve has failed.	None needed
721	Safety hazard	The control thinks it senses a flame even though the burner is supposed to be off.	None needed
741	Pro required	The water heater and its plug in remote control panel are not communicating with each other correctly.	1 safe check Look at the home's breaker panel to see if the breaker for the water heater has tripped
751	Pro required	The water heater and its built in control panel are not communicating with each other correctly.	1 safe check Look at the home's breaker panel to see if the breaker for the water heater has tripped
761	Pro required	In a system with more than one linked water heater (Easy-Link), the main unit and a connected unit are not communicating correctly.	1 safe check Look at the home's breaker panel to see if the breaker for the water heater has tripped
941	Safety hazard	On indoor models, the exhaust gas leaving the unit is hotter than it should be, or in a recirculation system the inlet and return water are both hotter than 140 degrees F.	1 safe check Look at the outside vent pipe for blockage such as snow, ice, leaves, or an animal nest
991	Safety hazard	The burner is not burning gas cleanly and completely, most often because of a blocked vent or not enough combustion air.	2 safe checks Look at the outside vent pipe and fresh air intake pipe for blockage Look at other gas appliances in the home to see if they are working

Sources

AO Smith Maintenance Sheet 5M2054-1, model 540P (P3H-T) tankless water heater, error codes and wiring diagram (accessed 2026-07-03)

AO Smith tankless (on demand) gas water heaters, older 305 and 505 outdoor model family (VA2520WD, VA2532WD), 2 digit numeric codes plus a letter code (LC) on the plug in remote control digital display

Code	Severity	What it means	Safe checks
02	Pro required	The unit is only running its freeze protection heaters and will not fire the burner. The manual lists this as a service call condition.	None needed
03	DIY OK	Power to the unit was interrupted while a bath or tub was filling, so water will not flow when power returns. The manual states this fix applies only when the code appears after a power interruption during a bath or tub fill. Turn off all hot water taps, then press the ON/OFF button on the remote controller twice.	1 safe check Make sure all hot water faucets in the home are turned off before resetting the unit
10	Safety hazard	Something is blocking the air intake or exhaust vent, or the vent was not installed correctly.	1 safe check Look at the outside vent pipe and the fresh air intake pipe for blockage such as snow, ice, leaves, or an animal nest
11	Safety hazard	The burner flame failed, either it would not light or it went out.	1 safe check Look at other gas appliances in the home to see if they are working. This tells you if the home's gas supply is on
12	Safety hazard	The burner flame failed while the water heater was running.	1 safe check Look at other gas appliances in the home to see if they are working. This tells you if the home's gas supply is on
14	Safety hazard	A one time use thermal safety fuse has blown, usually because of restricted airflow, low water flow, or a blocked heat exchanger.	None needed
16	Safety hazard	The water heater's temperature has gone above its normal operating range.	None needed
32	Pro required	The sensor that reads the temperature of the water leaving the unit has failed.	None needed
33	Pro required	The sensor that reads the heat exchanger's outgoing water temperature has failed.	None needed
34	Safety hazard	The sensor that reads the temperature of the incoming combustion air has failed.	None needed
52	Safety hazard	The signal to the modulating gas valve, which adjusts flame size, is abnormal.	None needed
61	Safety hazard	The combustion fan has failed.	1 safe check Look at the outside vent pipe for blockage such as snow, ice, leaves, or an animal nest
65	Pro required	The motor driven water flow control does not stop water flow properly.	None needed
71	Safety hazard	One of the internal gas solenoid valves has an electrical circuit fault.	None needed
72	Safety hazard	The sensor that detects the burner flame has failed.	None needed
LC	Pro required	Mineral scale has built up inside the heat exchanger, restricting water flow and heat transfer.	None needed
No Code	Pro required	Nothing happens when a hot water tap is opened. Causes listed in the manual include a clogged inlet filter, crossed hot and cold lines, not enough flow to trigger the unit, or an electrical fault.	1 safe check Try opening more than one hot water faucet at the same time, some tankless units need a minimum combined flow before they will turn on

Sources

AO Smith Maintenance Sheet U287-1193(00), model 305 and 505 outdoor tankless water heater, error codes, remote controller, and wiring diagram (accessed 2026-07-03)

AO Smith standard atmospheric vent (standing pilot) residential gas tank water heaters with FVIR flammable vapor resistant construction, models GCV, GCVH, GVR, GCVL, GVRL, GCVT and XCV, XVR, XCVL, XGV, XVRL, XCVT. This is NOT a power vent unit, there is no blower, only a single status light near the gas control knob. Note this system is powered by the pilot's own thermopile, not household electricity

Code	Severity	What it means	Safe checks
0 flashes (LED not lit)	Safety hazard	The pilot light is out, or the control has no power.	2 safe checks Look through the sight glass viewport to see if the pilot flame is lit Look at other gas appliances in the home to see if they are working. This tells you if the home's gas supply is on
Status light on solid (not blinking)	Safety hazard	The pilot light was recently put out and the thermopile that powers the status light is still cooling down. It can take up to about 90 seconds of the pilot burning before the light starts to blink again.	1 safe check Look through the sight glass viewport to see if the pilot flame is lit
1 flash (bright/dim heartbeat)	Normal	This is normal operation. No problem is present.	None needed
2 flashes	Pro required	The pilot is lit, but the thermopile that generates power for the safety valve is producing a weaker signal than it should.	None needed
4 flashes	Safety hazard	The water temperature went above the safety limit and the energy cut off (ECO) device shut the gas off.	None needed
5 flashes	Pro required	One of the control's temperature sensors has failed.	None needed
7 flashes	Pro required	The gas control valve's internal electronics have failed.	None needed
8 flashes	Safety hazard	The gas control valve is not shutting the gas off properly when it is turned to the off position.	None needed

Sources

AO Smith Service Handbook 327726-001, Residential FVIR Gas Water Heaters, models GCV/GCVH/GVR/GCVL/GVRL/GCVT and XCV/XVR/XCVL/XGV/XVRL/XCVT Series 300/301, Diagnostic Flash Codes section (accessed 2026-07-03)
AO Smith GCV300 Service Handbook, diagnostic flash code table (corroborating source) (accessed 2026-07-03)

AO Smith power vent and power direct vent residential gas tank water heaters with the Intelli-Vent electronic ignition control, including GPDH, GPDT, and GPDX gas models with hot surface ignition. Codes are shown as a specific pattern of lit indicator lights (marked with a triangle pointer and letters A, B, C) on the Intelli-Vent control board, numbered 1 through 16 in AO Smith's troubleshooting chart. To read your unit, compare the lit pattern on the control to the numbered chart printed on the water heater or in the manual

Code	Severity	What it means	Safe checks
1	Pro required	The control is not sensing a proper earth ground connection for the ignition system.	1 safe check Look at the wall outlet the water heater is plugged into to see if it is a grounded, 3 prong outlet
2	Pro required	There is a wiring problem or high resistance in the earth ground, which can also show up as reversed electrical polarity.	None needed
3	Safety hazard	The air pressure switch stayed closed for more than 5 seconds after the water heater called for heat, so the blower does not start.	1 safe check Look at the outside vent pipe for blockage such as snow, ice, leaves, or an animal nest
4	Safety hazard	The air pressure switch stayed open for more than 5 seconds after the blower turned on. The blower may keep running continuously.	1 safe check Look at the outside vent pipe and fresh air intake pipe for blockage such as snow, ice, leaves, or an animal nest
5	Safety hazard	The control has detected an open circuit in the hot surface igniter that lights the burner.	1 safe check Look at other gas appliances in the home to see if they are working. This tells you if the home's gas supply is on
6	Safety hazard	The burner failed to light after repeated tries and the system has locked out for one hour for safety.	2 safe checks Look at other gas appliances in the home to see if they are working. This tells you if the home's gas supply is on Look at the home's breaker panel to see if the breaker for the water heater has tripped
7	Safety hazard	A self test found a problem in the circuit that drives the gas valve.	None needed
8, 9	Pro required	A self test found a problem with the control's internal microprocessor or other internal circuits.	None needed
10	Safety hazard	The control sensed the burner flame signal at the wrong point in the ignition sequence.	None needed
11	Safety hazard	The water in the tank went above 195 degrees F and the energy cut off (ECO) safety device activated.	None needed
12	Pro required	One or both of the temperature adjustment buttons on the control appear to be stuck.	None needed
13	Pro required	The water temperature sensor (thermistor) is either disconnected or short circuited.	None needed
14	Safety hazard	The flammable vapor sensor, part of the safety system that watches for gasoline or solvent fumes near the unit, is disconnected or short circuited.	None needed
15	Safety hazard	The flammable vapor sensor has detected fumes from something like gasoline, paint, or solvents near the water heater, and the control has locked out for safety. The manual states a qualified service person must perform the reset procedure after the hazard is removed.	1 safe check Look around the base of the water heater for open containers of gasoline, paint, paint thinner, or other strong smelling chemicals, and move them to a different area if found
16	Safety hazard	The unit has locked out because lint, dust, or oil buildup is repeatedly interrupting the air pressure switch signal during a heating cycle.	1 safe check Look at the outside vent pipe for blockage such as snow, ice, leaves, or an animal nest

Sources

AO Smith Technical Bulletin TB-A028-06, FVIR Intelli-Vent Troubleshooting Chart (accessed 2026-07-03)
AO Smith GPDH/GPDT/GPDX Gas Models with Hot Surface Ignition, Residential Gas Water Heater Owner's Manual, part 184707-001, Trouble Shooting LED status table (cross confirms codes 1 through 13) (accessed 2026-07-03)

Last updated 2026-07-03.