Siemens LME11 / LME21 / LME22 fault 4: Extraneous light when burner is started up
Four red blinks mean the flame detector reported a flame at startup, before the burner control had opened any fuel valve. A startup precondition is that the flame detector is darkened and sees no extraneous light. Extraneous light during the prepurge time (t1) gives a non-alterable lockout; during the waiting time (tw) the start is prevented and a lockout follows after a maximum of 30 seconds. During startup the signal lamp shows green-red for extraneous light.
Diagnose this with AI
Try Free Boiler Diagnostic Upload a keypad photo, the fault history or a parameter export from your Siemens LME11 / LME21 / LME22. InduQT compares the evidence and suggests the next useful checks.
When it happens
- Waiting time (tw), before the fan starts
- Prepurge time (t1)
Where it sits in the burner
Supervised function: Flame supervision.
| Component | Signals to look at | How it fails |
|---|---|---|
| Flame rod | Flame signal | Scanner no longer sighted on the flame root; Flame rod or burner ground fault |
| Flame scanner (UV / IR) | Flame signal | Scanner no longer sighted on the flame root; Detector or amplifier ageing; Electrical noise on signal wiring; Dirty or fogged scanner sight path |
| Flame amplifier / flame input | Flame signal | Detector or amplifier ageing; Electrical noise on signal wiring; Wrong detector, amplifier or configuration |
| Main safety shutoff valves | Main fuel valve output, Valve proof of closure | Valve seat leakage or slow closing; Fuel valve not opening fully; Welded, jumpered or miswired contact |
Likely causes and checks
1Real flame or glowing fuel after shutdown
- Points to it
- Flame or afterburning visible in the combustion chamber; lockout follows a shutdown.
- Check
- Look into the combustion chamber; if flame persists after the valves close, isolate the gas and have the fuel valves tightness-tested by a qualified technician.
2Ignition spark arcing to the ionization probe
- Points to it
- Probe and ignition electrode close together, or electrodes recently bent or replaced.
- Check
- The data sheet says to locate the ignition electrode and probe so the spark cannot arc over to the probe; check the gap and position against the burner manual.
3Aged or faulty UV detector (QRA)
- Points to it
- UV-supervised burner; signal present in a cold, dark chamber.
- Check
- Cover the QRA and check whether flame is still reported; the data sheet describes a UV test of aged cells after controlled shutdown. Replace a detector that sees light that is not there.
4Interference on the detector cable
- Points to it
- Detector cable run together with mains or ignition cables, or longer than permitted.
- Check
- The data sheet requires a separate detector cable, never run with other cables, within the permitted length; reroute it.
Telling it apart
- 4 blinks is flame where none should be, at startup. 2 and 7 blinks are no flame where one should be.
- A green-red signal lamp during startup is the live indication of extraneous light; 4 blinks is the stored cause after the lockout.
Evidence to collect
- Blink count after pressing reset for more than 3 seconds
- Whether flame is visible in the chamber with the burner off
- Flame detector type (ionization probe or QRA)
- Detector cable routing and length
- Lamp colour seen during the start (green-red)
Work on fuel trains, burners, pressure vessels and electrical equipment must be done by qualified personnel following the manufacturer’s documentation and local codes. Never bypass a safety device or interlock.