Weil-McLain A01 Error Code — What It Means
The A01 error code is a lockout condition that appears when the boiler control module fails to detect a flame during the ignition sequence. The boiler attempts to light (typically after flashing a d3 code to show an ignition attempt is underway), but the flame sensor never confirms successful ignition. The system shuts down and locks out to prevent unburned gas from accumulating.
This code always follows a failed ignition cycle. It does not mean the control board is broken or that major components are damaged. It simply means the conditions for combustion were not met, and the boiler needs troubleshooting before it can restart.
Before You Replace Anything
Homeowners often replace the gas valve or control board when the real problem is a dirty or misaligned ignition electrode. Inspect and clean the ignitor and flame sensor first.
Common Causes
- No gas supply or air in the line (~35%) Common in new installations where the gas line was not purged of air, or the supply valve is closed or partially open.
- Dirty or misaligned ignition electrode (~25%) Carbon buildup, physical damage, or incorrect positioning prevents the ignitor from sparking or the flame sensor from detecting combustion.
- Faulty gas valve (~15%) The valve fails to open or does not deliver gas even when the control board signals it to open.
- Insufficient gas supply pressure (~10%) Gas pressure below the required 3.5 to 11 inches water column prevents proper ignition.
- Blocked condensate trap (~10%) A blocked trap or incorrect double trapping without a loop causes back pressure that interferes with combustion air.
- Air trapped in the heat exchanger (~5%) Air pockets cause gurgling noises and higher internal temperatures that can overheat the ignitor or prevent proper flame detection.
Quick Diagnosis
Answer these to narrow it down fast.
Does the ignitor produce a visible spark when the boiler attempts to start?
No: The ignitor may be dirty, damaged, or the control board is not sending power. Inspect and clean the electrode first.
Do you hear gurgling or sloshing noises when the boiler starts?
No: Check the gas supply valve position and condensate trap for blockages.
Is this a new installation or has the gas line been recently worked on?
No: Inspect the ignition electrode, flame sensor, and condensate trap for dirt or blockages.
Step-by-Step Fix
- Turn off power and gas to the boiler at the main shutoff and electrical disconnect for safety.
- Verify gas supply by checking that the gas valve handle is parallel to the pipe (open position) and that upstream shutoffs are open.
- Inspect the ignition electrode and flame sensor by removing the access panel and checking for carbon buildup, cracks, or bent metal. Clean with a soft brush or fine sandpaper if dirty.
- Check gas supply pressure using a manometer connected to the test port on the gas valve. Pressure must read between 3.5 and 11 inches water column.
- Inspect the condensate trap for blockages by disconnecting the drain hose and flushing the trap with water. Verify the hose is not looped and that double trapping is not creating back pressure.
- Purge air from the heat exchanger by opening the air bleed valves on the boiler while the circulator pump runs. Listen for gurgling to stop.
- Restore power and gas, then press the reset button on the control module for 1.5 seconds to clear the A01 lockout and observe the ignition sequence. If the boiler lights and runs without error, the repair is complete.
Parts Often Needed
| Part | Notes |
|---|---|
| Ignition electrode (Item 140) | Amazon | Verify your model uses Item 140 before ordering. Check the parts diagram in your manual. |
| Flame sensor rod | Amazon | Often integrated with the ignitor assembly. Replace if cracked or corroded beyond cleaning. |
| Gas valve | Amazon | Only replace after verifying gas supply, pressure, and ignition components are all working. |
When to Call a Pro
Call a licensed HVAC or boiler technician if you are not comfortable working with natural gas or propane systems. Gas work requires specialized tools (manometer, multimeter, combustion analyzer) and knowledge of local codes. A technician should handle all tasks involving gas pressure measurement, valve replacement, control board diagnostics, or heat exchanger inspection. If the boiler has locked out multiple times or you smell gas, shut off the gas supply immediately and call for service. Do not attempt to bypass safety controls or force the boiler to restart without identifying the root cause.
Rough cost: A pro service call runs about $150-350.