Furnace Flame Sensor: Microamp Readings, Cleaning and When to Replace

When a furnace lights and shuts off a few seconds later, flame sensing is the first suspect. The fix is often a cleaning, but measuring before and after tells you whether you actually solved it.

How flame rectification works

The control board puts an AC voltage on the flame rod. The flame conducts it to the burner (ground), and because the rod is much smaller than the grounded burner, the flame lets current flow better in one direction. That turns the AC into a small DC microamp signal. The board looks for that DC current as proof of flame. No signal, or too little, and it closes the gas valve.

How to measure

1. Set the meter to DC microamps (µA).
2. Put the meter in series: disconnect the sensor wire and connect the meter leads between the wire and the sensor terminal. Many meters and adapters make this easy.
3. Run a call for heat and read the current once the flame is established.

What's normal

• Readings of about 2–6 µA are common.
• Acceptable ranges run from about 0.5 to 10 µA depending on the control. Some manufacturers publish their own minimums.
• Many boards drop out somewhere below about 1–1.5 µA.

The board's minimum is in the service manual. The goal is a strong, steady signal, not one that barely clears the minimum.

Fixing a weak signal

1. Clean the rod. Remove the sensor and clean the rod with a fine abrasive pad (like a non-metallic scouring pad or fine emery cloth) until it's bright. Don't touch the rod with bare fingers, and wipe off any residue.
2. Check ground. The burner assembly must be grounded back to the board. Look for corrosion, loose screws or a bad ground wire.
3. Check polarity. Reversed hot and neutral on the line-voltage supply can kill flame sensing on many boards.
4. Check flame contact. The flame must cover the rod. Dirty burners, low gas pressure or a bent rod can leave it outside the flame.
5. Inspect the porcelain. A cracked insulator can leak signal to ground. Replace the sensor.

Measure again after each fix.

When to replace

Replace the sensor if the porcelain is cracked, the rod is badly worn, or the signal stays weak after cleaning while ground, polarity and flame are all good. If a new sensor still reads low, look at the board and the ground circuit.

FAQ

How often should a flame sensor be cleaned?

Check flame current at every heating maintenance visit and clean when it's dropping. Don't wait for no-heat calls.

Can I clean a flame sensor with sandpaper?

Coarse sandpaper can scratch the rod and leave grit behind. A fine, non-metallic abrasive pad or fine emery cloth is the usual choice.

Why does the furnace light and shut off after 3–7 seconds?

That's the classic sign of failed flame proving. The board didn't see enough flame current within the trial time, so it closed the gas valve.

Sources

• HVAC School: flame sensing basics
• Ferguson: furnace flame sensor troubleshooting guide
• PickHVAC: gas furnace flame sensor guide