Weil-McLain A11 Error Code — What It Means
Error code A11 (often displayed simply as ‘11’ on the control module) indicates an ignition lockout. The boiler attempted to light the gas burner multiple times but did not detect a flame via the flame sensor, so it shut down for safety. The control module stops and locks out to prevent unburned gas from accumulating in the boiler or venting system.
This is an urgent fault. The system will not restart until the underlying problem is corrected and the lockout is manually reset. In most cases, the issue is related to gas delivery, ignition components, or combustion air/exhaust problems.
Before You Replace Anything
Many technicians replace the control module first, but the real problem is usually a dirty flame sensor or closed gas valve. Clean the sensor with fine abrasive and verify the gas shutoff valve is open before ordering any boards.
Common Causes
- Gas supply interrupted or valve closed (~30%) The gas shutoff valve near the boiler is closed, gas pressure is too low, or the main gas supply to the building is interrupted.
- Dirty or failed flame sensor (~25%) The flame sensor is coated with carbon, cracked, or has failed electrically so it cannot confirm ignition.
- Ignition electrode fault (~20%) The spark electrode is misaligned, coated with carbon, or the ignition transformer is failing to produce a spark.
- Vent termination blockage (~15%) A blocked PVC flue (from ice or debris) prevents proper combustion air intake or exhaust, causing ignition failure.
- Gas valve or control relay failure (~10%) The gas valve relay is not opening or the gas valve modulation is faulty, preventing fuel from reaching the burner.
Quick Diagnosis
Answer these to narrow it down fast.
Is the gas shutoff valve near the boiler fully open?
No: Open the valve fully, wait two minutes for gas to reach the valve, then reset the lockout and observe if the boiler lights.
Is the system water pressure 12-15 PSI on the gauge?
No: Add water via the fill valve until pressure reaches 12-15 PSI, then reset the boiler and observe.
Can you see or hear a spark when the boiler tries to ignite?
No: No spark means the ignition transformer, electrode, or control module has failed. Call a licensed technician.
Step-by-Step Fix
- Turn off power at the boiler switch or circuit breaker and close the gas shutoff valve near the boiler.
- Check gas supply by verifying the shutoff valve is open and testing gas pressure at the valve inlet with a manometer (requires a licensed gas fitter).
- Verify system water pressure is 12-15 PSI on the boiler gauge. Add water via the fill valve if below 10 PSI.
- Inspect the external flue termination (PVC pipe exit) for ice, snow, or debris. Clear carefully without damaging the pipe.
- Remove and clean the flame sensor by unscrewing it from the burner assembly and wiping the metal rod with fine-grit abrasive or a dollar bill to remove carbon buildup.
- Check the spark electrode for alignment and gap per the manual. Clean any carbon deposits or replace if cracked.
- Test electrical components with a multimeter: verify 24 VAC at the gas valve coil during ignition attempt and 10,000-15,000 V at the ignition transformer output.
- Reset the lockout by pressing the reset button on the control module once, then observe the ignition sequence. If it fails again, replace the faulty component.
Parts Often Needed
| Part | Notes |
|---|---|
| Flame sensor (Weil-McLain) | Amazon | Verify your model number before ordering. The sensor is often model-specific. |
| Ignition electrode or spark rod | Amazon | Match the electrode to your boiler series (Ultra, ECO, CGa, etc.). |
| Gas valve (24 VAC) | Amazon | Requires a licensed gas fitter to install and test. Order by exact model number. |
When to Call a Pro
Call a licensed gas fitter or HVAC technician immediately if you cannot restore gas flow, if the flame sensor is clean and the boiler still locks out, or if you see no spark during ignition. Any work on the gas valve, ignition transformer, control module, or pressure switch requires specialized tools and gas certification. Do not attempt to bypass safety interlocks or restart the boiler repeatedly, as unburned gas can accumulate and create an explosion hazard.
Rough cost: A pro service call runs about $150-300.