Siemens LME11 / LME21 / LME22 fault 5: Air pressure switch time supervision
Five red blinks mean the air pressure switch was already in its working position when the burner control expected it at rest. A startup precondition is that the air pressure switch is in its no-load position; if it is welded in the working position, startup is prevented and a non-alterable lockout follows after 65 seconds.
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) and start prevention, before the fan is switched on
Where it sits in the burner
Supervised function: Combustion air proving.
| Component | Signals to look at | How it fails |
|---|---|---|
| Combustion air proving switch | Combustion air proving switch, Combustion air pressure | Pressure or position switch out of adjustment; Welded, jumpered or miswired contact; Restricted combustion air path |
| Combustion air fan (FD / ID) | Combustion fan running, Combustion fan speed, Combustion air pressure, Fan motor current, Fan vibration | Restricted combustion air path; Fan motor or drive fault; Bearing wear or misalignment |
| Burner management controller | Burner sequence state, Controller supply voltage | Controller internal fault; Supply voltage, frequency or grounding problem; Configuration or commissioning data changed; Electrical noise on signal wiring |
Likely causes and checks
1Air pressure switch contacts welded in working position
- Points to it
- Lockout on every start after about a minute; the switch reads made with the fan stopped.
- Check
- With power isolated and the fan stopped, check the switch contacts for continuity and replace a welded switch.
2Short circuit on the air pressure switch wiring
- Points to it
- Damaged cable, or work on the switch or plug-in base.
- Check
- The data sheet warns to check the air pressure switch connecting lines for short circuits (terminals 3, 6 and 11); a qualified electrician should check them with power isolated.
3Air still moving with the fan off
- Points to it
- Intermittent, linked to strong chimney draught, wind or another fan sharing the flue.
- Check
- Measure the pressure at the switch sensing point with the fan off and compare with the switch setting.
4Fan running when it should be off
- Points to it
- Fan audible during the waiting time.
- Check
- Check the fan motor supply and any external contactor that could keep the fan running.
Telling it apart
- 5 blinks is the switch made when it should be open. 3 blinks is the switch not made (or lost) when air is needed.
Evidence to collect
- Blink count after pressing reset for more than 3 seconds
- Time from call for heat to lockout (about 65 s points to start prevention)
- Switch state with the fan off
- Recent work on the air pressure switch or its wiring
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.