The flame rod (flame sensor) detects flame presence via micro-current. Carbon and oxidation buildup degrades the signal. The most-replaced Bosch part, behind a large fraction of Code A1 and Code A4 faults on units 5+ years old.
Symptoms
- Intermittent Code A4 (flame failure mid-operation)
- Persistent Code A1 on older units
- Unit takes multiple attempts to light
- Burner cycles on/off during a single draw
Clean first (free)
- Power off and shut off gas
- Remove the front cover
- Locate the flame rod — small metal probe in burner area
- Unscrew the rod
- Clean with fine sandpaper (400-grit) or Scotch-Brite. Polish until bright metal
- Reinstall, close, power on, test
Cleaning resolves about 60% of cases on units 5+ years old.
Replace if cleaning doesn't work
Bosch flame rods are model-specific. Cost: $25-70 OEM.
Bottom line
Before calling anyone about Code A1 or A4, clean the flame rod. Free, 20 minutes, most common fix.