Flame sensor microamp check

What is going on

Flame current is measured in series with the sensor. The integrated ignition control needs 1 to 4 microamps to prove flame. Contaminated fuel or air can coat the sensor and reduce the signal.

Likely causes

• High resistance wiring connections
• Sensor-to-burner gap wrong
• Dirty flame sensor
• Poor grounding
• Failed ignition control (no reading after continuity check)

Check in this order

1. Shut off power and disconnect the flame sensor wire from the sensor.
2. Connect a micro-amp meter in series between the wire and sensor terminal, positive to the wire and negative to the sensor.
3. Restore power and start a heating cycle.
4. Read the micro-amp value once flame is established. The hot surface ignitor turns off after proof of flame.
5. If the reading is below 1 microamp, check wiring, the sensor-to-burner gap, sensor cleanliness and grounding.
6. If there is no reading, check continuity of all components, then replace the ignition control if good.
7. Clean a coated sensor with steel wool.

More furnace guides

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• Alternate combustion air opening (upflow or horizontal)
• Alternate vent/flue location (upflow or horizontal left)
• Annual inspection checklist
• Auxiliary limit control (location and purpose)
• Auxiliary limit switch open (blower compartment)