What is the CORRECT procedure to diagnose and clean a flame sensor?
The flame sensor does not contain any moving or functional parts. The flame sensor is energized with AC voltage, which is rectified to DC in the presence of flame. The DC current flow is measurable in micro amps (uA). A minimum micro amp measurement is required to continue the heating cycle. On the 33” gas furnaces, 3.7 uA is a typical measurement, with 1.5 uA resulting in a “low flame signal” flash code, and measurements less than .5 uA result in interruption of the heating cycle.
On many models, the flame current may be read at the flame current pad built into the furnace control board. The control board converts the uA measurement to DC voltage (1 uA = 1 volt DC, see attached illustration).
Alternately, on furnaces that do not have a flame current pad, the micro amp measurement may be read by placing a meter set to DC Amps in series with the flame sensor (see attached illustration).
If the flame signal is not adequate for flame proof, check the following:
- A dedicated ground wire must be connected to the furnace
- All wiring connections must be tight
- Manifold pressure must be correct per the furnace rating plate
- The flame must be impinging on the flame sensor
- The flame sensor must be clean and free of oxidation. If necessary to clean, use extra fine steel wool. Cleaning the flame sensor is considered normal maintenance, and is not covered under warranty. If corrosion or contamination is severe enough that cleaning with steel wool will not improve the sensors performance, the flame sensor must be replaced.
- Porcelain on the flame sensor must be intact. If the ceramic insulator is cracked, the flame sensor must be replaced.
- Check to see if there is voltage at the sensor. Without a call for heat, there should be voltage (approximately 90 to 110 volts) between the sensor and ground. The sensor is always looking for flame and is always energized. If no voltage output is found, check the wiring to the board or replace the board if no voltage output is present.