CTO Troubleshooting


There are a few different reasons that can cause the gas burner to not light.

Following these steps may diagnose the problem.

During normal operation with the burner operating correctly, there are a few sounds that can help diagnose certain problems. Normally, when the thermostat is turned up, the burner blower will begin blowing air into the firebox and about 40 seconds later, the gas valve will click and the igniter will buzz. If the flame does not light, the buzzing will stop after about 5 seconds and the ignition process will stop. This will not repeat unless the thermostat is turned off and then back on.

If the burner blower does not start blowing, nothing else on the burner will work. If this is the case follow the steps to diagnose a faulty thermostat or a Hi Limit that is tripped.


The thermostat is the control device that controls the temperature and sends the power to the burner. If the contacts inside the thermostat fail to connect, the burner will not get power to it.

The thermostat itself is located inside the control box. With the cover off of the control box, the thermostat wiring can be accessed.

The thermostat is the rectangular metal box with the X on it. The two wire connections are at the top.

If a voltmeter is available for testing purposes follow these steps:

With the thermostat turned to the “OFF” position, there should be 120 V AC checking from one screw to a ground.

In the “ON” position and high enough to call for heat, there should be 120 V AC on both screws, checking one at a time to a ground.

If there is 120 V AC on one screw only with the thermostat turned up high enough to call for heat, the thermostat is failing.


If a voltmeter is not available, there is another option for diagnosing a defective thermostat.

Unplug the power cord to eliminate the risk of electrical shock.

Loosen one of the screws that connect the thermostat wires to the thermostat and remove one of the wires. Loosen the screw on the other side and add the second wire in with the first. This will have both wires together under the same screw. Tighten the screw.

Plug the power cord back in and try the burner again. If the burner now comes on, the thermostat is not working and will need to be replaced. While the burner will burn with the wires connected together, there will be no temperature control and the temperature will continue to rise until the hi limit switch turns it off.


If the thermostat tests out good and the burner still does not light, the Hi Limit may be tripped. The reset button is located on the back side of the control box. Locate this button and press it in. If it is tripped, there will be a click when it is pressed. This will have it reset and allow the burner to work again. If the Hi Limit does not move in or does not click, it was not tripped.


If the burner blower is blowing, the following sounds should be heard. After about a 40 seconds after the burner blower starts, the gas valve should click and the igniter should buzz for a few seconds. If you don’t hear either of these sounds, the ignition module may be failing. The two yellow wires plug in to the ignition module at terminals marked 25V Ground and 25 V. Unplug these two wires and check across the two wires with a voltmeter with the burner blower running. There should be 25V AC present across the two wires. If a 25V reading is present on the voltmeter, the ignition module has failed and must be replaced.

*Refer to service department for exact model.

If the 25 V is not present across the two yellow wires and the burner blower is blowing, the centrifugal switch built in to the burner blower may not be working. This is a safety switch built in to the burner blower so the gas function will not work if the burner blower does not come on or does not reach normal speed. Sometimes this micro switch button cab be lubricated with light weight oil and get it to start working. Normally, the spring arms pull the white nylon cam away from the micro switch button and allow the button to move outward. If the button sticks in, the burner does not know the blower is running.

Remove the two nuts from the end of the cover and pull the cover straight off.

Move the white nylon cam to access the red micro switch button. Using a small screwdriver, you should be able to push the button in a slight amount. If it is stuck in and does not move try oiling with light oil like WD40.

If this is not successful producing the 25 V across the two yellow wires, the centrifugal switch is bad and the burner blower must be replaced.


A Flame Sensor electrode is located inside the burner tube. This is a safety device that allows the gas to stay on only if the flame sensor detects the heat from the flame. This flame sensor is located at the bottom of the burner tube.

This flame sensor is in the inside of the burner tube where the flame comes in to the fire box a few inches back from the end. It is possible for ashes to get into this burner tube and cause the flame sensor to fail. Blowing compressed air back into the burner tube from the fire box chamber can clean these ashes out and cause the flame sensor to work again. Compressed air in a can can be used as long as there is no fire or embers in the fire box. The propellant in these cans may be propane and could catch fire.

If this does not correct the problem, the flame sensor may need to be replaced. The following pictures will help show the steps to access the flame sensor for replacement.

Unplug the orange wire and remove the ignition module cover. Unplug the blue wire after the cover is removed. Unscrew the ignition module bracket so the ignition module can be moved.

Remove the 4 screws that hold the gas train to the front of the burner plate. This will allow the gas train to be moved to the side.

Remove the 4 screws from the corners of the burner plate. Separate the two burner plates with a screw driver. Pull the inner burner tube straight out.

The blue wire is plugged in to the flame sensor.

The flame sensor is held in place with a screw and nut. The new flame sensor will position itself correctly when the screw and nut are replaced. Be careful when inserting the inner tube back in to the burner when reassembling so the flame sensor does not get bent out of position.

Sometimes the function of the flame sensor can be improved by making slight adjustments to the air shutter on the burner blower. The small screw at the top holds the air shutter in place. The setting of the air shutter is important to control the proper ratio of air and gas but minor adjustments can be made to help improve the efficiency of the flame. Normal settings are between 1 and 2 on the numbered scale.