The flame sensor is a metal rod in the burner area that detects flame presence by electrical rectification. Flame conducts a tiny DC current from the rod to ground (the burner shell). If the control sees current, flame is present; no current = ignition trial failed.
Symptoms of dirty/failing sensor
- E04 ignition lockouts
- Flame lights briefly then goes out (control thinks ignition failed even though flame was present)
- Multiple ignition trials needed before flame establishes
The annual-service cleaning
Cleaning the flame sensor is part of every annual service. Procedure:
- Power off; gas off
- Remove burner cover
- Locate the flame sensor (rod near burner)
- Carefully remove from bracket (single screw or clip)
- Polish surface with fine sandpaper (220-400 grit) or steel wool — removes oxidation
- Wipe with clean cloth
- Reinstall in same position (orientation matters)
- Restore gas and power; verify clean ignition
When to replace vs clean
- Clean if oxidation visible but rod is straight and intact
- Replace if rod is bent, burned, eroded, or cracked
- Replace if cleaning doesn\'t resolve E04 lockouts
Cost
- Flame sensor (OEM): $25-50
- Cleaning is part of annual service (no extra cost)
Bottom line
Cheapest and most common Lochinvar repair. Dirty sensor is the #1 cause of E04 lockouts on units that have skipped annual service. 5-minute fix; bundle with annual service or do as part of any E04 diagnostic.