Troubleshooting should only be performed by a trained technician in accordance with all local, state, and national codes and licensing requirements. There is an extreme risk of personal injury or property damage when performing service procedures.
The remote flame sensor monitors the system for burner ignition and operation. It mounts over the burner and is located in the flame when the burner is lit.
The sensor monitors the flame through the process of flame rectification. The sensor is energized from the AC line voltage circuit of the furnace (115-120 volts in the case of a gas furnace). The flame conducts a small electrical current through the flame to the chassis ground of the furnace. This current is measured in microamps and is very sensitive to cleanliness and grounding issues.
The microamp current is dependent on the control board, the contact area with the flame, and the integrity of the chassis ground path between the burner and the control board. The furnace chassis must have a solid ground connection. The polarity of the incoming power supply must also be correct. If the hot and neutral power supply wires are reversed the furnace will not sense flame. Some furnaces have a “flame” light on the control board that lights up to indicate proper flame sense current and flicker when the signal is marginal.
On most furnaces the gas valve will open and the igniter will light the burner for a short time, usually 8-10 seconds. If flame is not detected during the trial ignition period, the furnace will repeat the cycle. After a set number of ignition trials the furnace will lock out. Depending on the ignition system, the lock out period may reset after an hour, or may require that power to the furnace is broken to reset the failure.
Flame sensors can become corroded or dirty. Since the flame sense current is so faint, the furnace will not sense flame. Flame sensors can be cleaned with steel wool or a scrubbing pad (without soap). Many technicians use a dollar bill to scrub the flame rod. A sandpaper-type product should never be used on a flame sensor. The abrasive residue will melt in the flame creating a glass-like coating on the flame rod which ruins it.
Common causes of flame rectification failure include:
A hot surface igniter has a ceramic mounting base internally connected to a heating element made of either silicon carbide or silicon nitride. The element glows red hot when voltage is applied which ignites the gas. Once the gas is ignited, the voltage to the igniter is de-energized. The heating element is extremely brittle and easily broken. It is a long-held industry belief that handling an igniter by the element with bare hands can create a hot spot from the oil on your skin causing igniter failure.
The first step in troubleshooting is to do a visual check. In the most cases the igniter will have an obvious burned spot or gap in the element.
Next, make sure that there is voltage flowing to the igniter. To check this, disconnect the igniter, establish a call for heat, and check for voltage (115-120 volts AC for a gas furnace) at the connector going to it. If there is no voltage going to the igniter, then the problem lies elsewhere.
If the igniter is intact and has proper voltage going to it, the next step is to check the resistance with an Ohmmeter. In general, the resistance will be somewhere from 40 to 250 Ohms, depending on the igniter. In practical terms, the igniter will either have resistance (good), or have no resistance (open).
In simple terms, if the igniter has voltage going to it but does not glow, it is defective and should be replaced.
by OneTrip Parts®