CSLB C-20 HVAC ContractorHeating SystemsMedium
A technician is troubleshooting a furnace that ignites briefly then shuts off after a few seconds, displaying a flame sensing fault code. Testing the flame sensor with a microammeter during burner operation shows only 0.5 microamps DC, below the minimum required. What does this most likely indicate?
- AThe high-limit switch is stuck open
- BThe transformer secondary voltage is too low
- CThe flame sensor rod is coated with oxidation and needs cleaning
- DThe gas valve regulator is set too high
Show answer & explanationAnswer & explanation
Correct answer: C. The flame sensor rod is coated with oxidation and needs cleaning
Flame rectification sensors rely on a clean metal rod contacting the flame to generate a weak DC microamp signal that proves flame presence to the control board. A carbon or oxide coating insulates the rod, reducing the signal below the control's minimum threshold (typically 1-2 microamps), causing nuisance lockouts. Cleaning the rod with fine emery cloth restores proper current flow.
Why the other options are wrong
- A. A stuck-open high limit would prevent the blower or shut down heat differently, not produce a low flame-sense reading.
- B. Low transformer voltage would affect gas valve or control operation broadly, not specifically the microamp flame signal.
- D. Gas pressure issues would affect flame size/quality but not directly cause a low microamp reading from sensor coating.
Flame Rectification
A safety method where a flame sensor rod uses the flame's ionized gases to allow a small one-directional DC current to flow, proving flame presence to the control board.
- Typical minimum signal is 1-3 microamps DC
- Oxidation/carbon buildup is the most common failure cause
- Clean with fine emery cloth, not sandpaper (can leave residue)
Memory trick: Dirty rod, dead current, dead flame.