A.O. Smith Vertex Error Codes: What the Messages Mean

By Ray Bennett · Updated 2 October 2026 · 5 min read · Checked against the maker’s manual

A.O. Smith Vertex Error Codes: What the Messages Mean

Short answer

The A.O. Smith Vertex shows its faults in plain words, and locks out until the cause is fixed. A.O. Smith's reset is the power off for about 20 seconds, then on. The ones you can check: Blocked Exhaust, which its own screen says to check by making sure the condensate hose drains, Blocked Air Intake, often a clogged intake screen, and Ignition Failure, starting with the gas valve.

The A.O. Smith Vertex is a high efficiency gas water heater with a combustion blower that pushes its exhaust out through a vent pipe. Instead of a blinking light, its control shows faults as plain words on a small screen, such as Blocked Exhaust or Ignition Failure, and locks out until the cause is dealt with.

A.O. Smith’s service handbook for the Vertex GDHE 50 Series 120 and 121 lists every message. This page sorts out the few an owner can check from the ones that need a technician.

Before you start

What to carry: a torch, a bucket and a pen. Fifteen minutes.

Gas or electric? Gas, natural or propane, with a 120 volt plug or supply for the control and the fan.

Read the whole message. A.O. Smith says the control keeps a history of the last nine faults with the time of each, and a count of how often each one has happened. A technician will want both.

Before any reset. If you smell gas. Leave the house and call from outside. Do not reset anything.

Energy Cut Out means very hot water. A.O. Smith says to check a nearby hot tap with care. Above 180°F, shut off the gas and the power and call for help, and do not leave the heater running.

A technician’s handbook. A.O. Smith says servicing should only be done by a qualified service agent. The checks below are the ones that need no tools inside the heater.

How to reset it

Check 1Free, one minute

Power off, then on

The reset. A.O. Smith says to turn the power off for about 20 seconds, then back on. If the cause has not been fixed, the control locks out again.

A blank screen. A.O. Smith’s first checks are the on and off switch, the plug, a tripped breaker or blown fuse, and power at the 120 volt outlet: how to reset a water heater.

Buttons that do nothing. A.O. Smith points at the ribbon cable from the button pad. That is a job for a technician.

The messages you can check

The Vertex messages with a check an owner can make, from A.O. Smith's own handbook.

Check 2

Check the vent, the intake and the gas

Blocked Exhaust. The screen itself says to make sure the condensate hose is draining. A.O. Smith says water building up in the exhaust elbow restricts the venting: clear the condensate drain line so it flows freely. A low point in the vent pipe that holds water does the same.

Blocked Air Intake. A.O. Smith says to check the screen inside the intake air connection for debris. On a direct vent install, the screen should have been removed at installation, and a low point in the intake pipe can hold water.

Ignition Failure. Three tries without a flame. A.O. Smith’s first checks: the main gas shut off valve is open, there is enough air for combustion, and the fuel matches the rating label: a gas water heater with no hot water.

AC Reversed. The power supply polarity is reversed. A.O. Smith says to check the outlet with a plug in tester, and the wiring in the junction box.

Energy Cut Out

A.O. Smith's steps for the Energy Cut Out message. The hot tap reading decides everything.

Check 3Free, or a service call

Handle the ECO carefully

What it means. A.O. Smith says the control has seen tank water above 201°F. The tank must cool to 140°F before the lockout can be reset.

Check the water. Carefully take the temperature at a nearby hot tap. Above 180°F, shut off the power and the gas and call. A.O. Smith says not to leave the heater running under any circumstances.

Below 180°F. Cycle the power to reset it, then watch several heating cycles. If the tank climbs more than 5°F past the set point, shut off the power and the gas and call.

The messages for a technician

Message	What A.O. Smith’s handbook says
Temp Probe Open or Short	The temperature sensor reads out of range
Flame Sensor Short	The flame sensor is grounded
Flame Detect Error	Flame seen out of sequence: the burner has not shut down properly
Blower Prover Failure	The blower prover switch closed before the blower started
Blower Prover Open	The switch stayed open after the blower was powered
Low Igniter Current	Less than 0.5 amps through the igniter during warm up
Communication Failure	The display and the control board cannot talk

Not enough hot water, and no message

Check 4

Check the settings and the demand

Too much demand. A.O. Smith’s first cause is the heater’s capacity being exceeded.

The settings. A set point too low, or a differential set too high. A.O. Smith says residential models go up to 160°F: what temperature to set.

The plumbing. Hot water valves not fully open, the inlet and outlet pipes connected the wrong way round, or a missing dip tube.

Questions people ask

How do I reset an A.O. Smith Vertex?

Turn the power off for about 20 seconds, then back on. A.O. Smith says it locks out again if the cause has not been fixed.

What does Blocked Exhaust mean on a Vertex?

The exhaust is restricted. The screen says to make sure the condensate hose is draining, and A.O. Smith says water in the exhaust elbow is a common cause.

What does Ignition Failure mean?

Three tries to light without sensing a flame. Check the gas valve is open, there is enough combustion air, and the fuel matches the label, then call for service.

What is Energy Cut Out on an A.O. Smith?

The tank went above 201°F. It must cool to 140°F before a reset. If a hot tap reads above 180°F, shut off the gas and power and call.

Why does my Vertex keep saying Blocked Air Intake?

A.O. Smith points at the intake screen, a screen left in place on a direct vent install, and water sitting in a low point of the intake pipe.

Can I see past faults?

Yes. A.O. Smith says the control keeps the last nine faults with a time stamp, and counts how often each one has happened.

Sources: A.O. Smith Service Handbook 318556-000 for the BTX 100 and the Vertex GDHE 50 Series 120 and 121 power direct vent gas water heaters