Weil-McLain A01 Error Code — What It Means
On Weil-McLain Aqua Balance boilers, code A01 (sometimes displayed as A1) means ‘No burner ignition.’ The control board called for heat and started the ignition sequence, but it did not prove flame within the allowed time window. The boiler then locked out to prevent unsafe operation.
The code tells you the burner failed to light, not which component is at fault. Common triggers include no gas reaching the valve, low inlet pressure, a fouled or misaligned ignition electrode, a stuck gas valve, a blocked condensate trap, or a dirty heat exchanger that chokes combustion air. In new installations the gas line may not have been purged, leaving air in the pipe.
Before You Replace Anything
Many people replace the control board first when they see A01. Before ordering electronics, verify that the ignitor sparks and that gas inlet pressure is within 3.5 to 11 in. w.c. A bad electrode or low gas pressure will mimic board failure but cost far less to fix.
Common Causes
- Low or missing gas supply (~35%) Gas pressure below 3.5 in. w.c. or air in the line on a new install will prevent ignition.
- Dirty or misaligned ignition/detection electrode (~30%) Carbon buildup, corrosion, or incorrect electrode gap blocks spark or prevents flame sensing.
- Blocked condensate trap or improper drain piping (~15%) A clogged trap or looped drain hose creates back-pressure that chokes the burner.
- Faulty gas valve (~10%) The valve coil may be open-circuit or mechanically stuck closed, blocking gas even when power is applied.
- Dirty heat exchanger or overdue maintenance (~7%) Soot or scale buildup restricts airflow and combustion, preventing stable flame.
- Air trapped in the heat exchanger (~3%) Unbled air pockets in the boiler or near-boiler piping disrupt flow and combustion.
Quick Diagnosis
Answer these to narrow it down fast.
Do other gas appliances in the home work normally?
No: Utility gas may be off or the meter shut. Call the gas company or check the main shutoff before troubleshooting the boiler.
Does the ignitor click or spark when you call for heat?
No: No spark suggests a bad ignitor, loose ignitor wire, or failed control board. Check connections first, then test or replace the ignitor.
Is the condensate trap full of water or debris?
No: Condensate path is clear. Check gas pressure, electrode condition, and heat exchanger cleanliness next.
Step-by-Step Fix
- Reset the boiler by holding the reset button on the control module for 1.5 seconds, then call for heat from the thermostat and watch the ignition sequence.
- Verify gas supply by confirming the manual shutoff valve upstream of the boiler is fully open and that other gas appliances light normally.
- Check inlet gas pressure using a manometer at the gas valve inlet tap. Weil-McLain requires 3.5 to 11 in. w.c. for proper operation.
- Inspect the ignition/detection electrode for carbon deposits, corrosion, or poor electrical connection. Clean or replace the electrode if fouled.
- Test the ignitor for spark by carefully draping it over the plastic control housing (not touching metal) and calling for heat. If spark is present, the board and ignitor are working and you should focus on gas delivery.
- Clear the condensate trap and drain of any blockage. Do not loop the condensate drain hose, run it in a straight downward path to the drain.
- Inspect and clean the heat exchanger if the boiler is overdue for maintenance or if combustion surfaces show soot or scale buildup.
- Bleed air from the heat exchanger and near-boiler piping if the system was recently filled or if the boiler is a new install.
- Verify vent length and configuration against the installation manual, ensuring long-sweep elbows are used where specified and total vent run is within limits.
- Replace the gas valve if gas pressure is correct, the electrode is clean, and the valve does not open when the control board sends power.
Parts Often Needed
| Part | Notes |
|---|---|
| Ignition/detection electrode | Amazon | Order by boiler model number. Clean first, replace only if cracked or heavily corroded. |
| Gas valve | Amazon | Model-specific. Test for coil continuity and mechanical operation before replacing. |
| Condensate trap assembly | Amazon | Replace if cracked or threads are stripped. Often cleanable rather than replaceable. |
| Control board | Amazon | Replace only after verifying spark, gas pressure, and electrode integrity. Expensive and rarely the root cause. |
When to Call a Pro
Call a licensed gas technician for A01 on a Weil-McLain boiler. Diagnosing no-ignition faults requires measuring gas pressure with a manometer, testing ignition spark safely, and inspecting combustion components inside the boiler cabinet. Gas work is regulated in most jurisdictions and mistakes can create carbon monoxide hazards or gas leaks. A qualified technician will also verify venting is correct, purge air from the system, and make sure the heat exchanger is clean and safe to operate. If the boiler is under warranty, have the installing contractor or a factory-authorized service company perform the repair to preserve coverage.
Rough cost: A pro service call runs about $150-350.