Dirty flame sensors drive more unnecessary board replacements than any other nuisance in residential gas furnace service. Before you pull out a $400 control board, run this.

What you're looking for

Microamp reading at the flame sensor with the burner lit. Spec varies by manufacturer but roughly:

Carrier / Bryant: 4–6 μA (shutdown typically at 0.7 μA)
Trane / American Standard: 2–4 μA
Lennox: 3–6 μA
Goodman / Amana: 3–5 μA

Anything below ~1.5 μA is marginal. At that level you'll see intermittent lockouts, delayed ignition, and the classic "runs fine when I test it, calls me back two weeks later" pattern.

Tools

True-RMS multimeter that reads μA DC
Fine steel wool OR emery cloth (600 grit)
#2 Phillips / 1/4" nut driver
Gloves (the sensor can be hot)

Procedure

Kill power. Switch at the furnace, not just the thermostat. Verify with your meter.
Shut the gas. Upstream of the valve.
Remove the sensor. Usually one screw. Pull straight out — don't twist the wire.
Inspect. Look for a dull film or a visible oxide buildup. Even a thin layer is enough to drop μA below spec.
Clean. Light passes with steel wool or emery cloth until the rod is bright, shiny stainless. Don't use sandpaper — silica grit embeds in the rod and creates a new insulator. Same reason don't use a wire wheel.
Wipe. Clean cloth to remove debris.
Reinstall. Torque to snug, no more — you'll break the porcelain.
Restore gas, restore power, call for heat.
Read the μA. Meter in series with the sensor wire (pull the spade, put meter between wire and sensor terminal). Read during steady burn, not during ignition.

If you're back in spec, you're done. If you're still marginal:

Check the ground. Flame sensing is a microamp-level rectification path. Any corrosion on the ground strap, the transformer secondary ground, or the burner assembly ground will kill μA. Ohm the ground path.
Check the sensor position in the flame. It needs to be in the blue cone, not just adjacent to it. Some sensors slip over time.
Check the porcelain. If you see a hairline crack, replace — it's leaking current to ground.
Replace the sensor. They're $15–$30. If cleaning gets you 2 months before it fails again, just replace.

Why this matters

Flame rectification works because DC passes through ionized combustion gas. The sensor acts as one terminal, the grounded burner assembly as the other. Anything that blocks electron flow at either end — oxide on the rod, bad ground, wrong position — drops μA.

The control board is doing its job correctly when it locks out on flame-sense failure. It's preventing a gas-flow-without-flame condition. Don't chase the board unless you've confirmed the sensing path is clean and grounded.

When to condemn the board

Sensor clean and in spec on bench but μA reads zero in furnace after verified ground
Board shows other unrelated anomalies (random resets, relay chatter)
Diagnostic LED pattern doesn't match documented codes for the control

CYA language

This walkthrough is for licensed technicians. Homeowners: don't. Gas-fired equipment mishandling causes fires and CO poisoning. If you're a homeowner reading this, talk to Dale instead — he'll help you describe the problem to a professional so you're not flying blind on the service call.

iHVAC is not responsible for field decisions made by technicians using this guide. Always verify against the equipment manufacturer's service literature and local code.

Flame Sensor Cleaning: Field Guide for Working Techs