Weil-McLain Boiler A64 Error — What It Means
Weil-McLain boilers with Aqua Balance or similar digital controls display numeric and alphanumeric error codes on the status panel. The exact meaning of an A64 code depends on your boiler model and control family. Weil-McLain’s own technical documentation confirms that error numbers are listed in each model-specific service manual, but the A64 definition is not shown in the general-purpose materials available here.
Because Weil-McLain produces many boiler lines (Ultra, Gold, GV, EG, Eco-Tec, Aqua Plus, and others) with different control platforms, the same code number can point to different faults on different models. To identify the exact fault, locate the boiler model number and serial or CP number on the rating plate, then consult the installation and service manual shipped with your unit or download it from Weil-McLain’s website. If the manual is not available, contact Weil-McLain technical support or a qualified boiler technician with your model and serial information.
Before You Replace Anything
Without knowing the exact A64 definition for your model, do not replace the control board or ignition module first. Check the boiler manual to map the code to the correct sensor, switch, or limit, then test that component before ordering any part.
Common Causes
- Model-specific sensor or limit fault (~40%) Each Weil-McLain control maps A64 to a particular temperature sensor, pressure switch, or high-limit that is unique to that boiler family.
- Wiring fault or loose connector (~25%) A loose plug at the sensor or control terminal can cause the board to log an open-circuit or out-of-range fault even when the sensor itself is good.
- Failed sensor or switch (~20%) If the code definition points to a specific sensor (outlet, inlet, flue, or outdoor), that sensor may have drifted out of range or failed open.
- Control board memory glitch (~10%) A brief power interruption or static discharge can log a spurious code that clears on reset and does not return.
- Incorrect manual or mismatched control (~5%) If a control was replaced with the wrong version or the manual downloaded does not match the installed board, the code table will not align.
Quick Diagnosis
Answer these to narrow it down fast.
Does the boiler display show a steady A64 with no other codes or flashing segments?
No: If the display cycles through multiple codes or flickers, you may have a wiring issue or a failing control board. Check all wire connections and measure AC line voltage at the boiler.
Does the code clear when you press reset and stay off for at least one full heat cycle?
No: The fault is repeatable. Use the manual's diagnostic table to test the component linked to A64 on your model.
Can you find the exact A64 definition in your boiler's installation and service manual?
No: Call Weil-McLain technical support at the number on the rating plate with your model and serial number, or hire a technician who has access to the service literature.
Step-by-Step Fix
- Turn off power to the boiler at the circuit breaker and wait 30 seconds for the control to clear.
- Locate the boiler rating plate on the jacket or burner door and write down the full model number, serial number, and CP number if present.
- Download the installation and service manual for that exact model from the Weil-McLain website or retrieve the paper manual from the boiler documentation packet.
- Look up code A64 in the error-code table, which will list the component name and diagnostic steps for that fault on your model.
- Restore power and observe whether the code returns immediately or only during a heat call, which narrows the list of possible causes.
- Test the component identified in the manual using a multimeter set to the range specified in the diagnostic table (resistance for sensors, continuity for switches).
- Call Weil-McLain technical support or a licensed boiler technician if the manual does not list A64 or if the test results are ambiguous.
Parts Often Needed
| Part | Notes |
|---|---|
| Weil-McLain temperature sensor (model-specific) | Amazon | Order by your boiler model and the sensor location (outlet, inlet, or flue) listed in the A64 definition. |
| Weil-McLain pressure switch or high-limit switch (model-specific) | Amazon | Check the manual to confirm whether A64 maps to a pressure or limit switch on your control before ordering. |
When to Call a Pro
Call a licensed boiler technician if you cannot locate the model-specific manual, if the A64 definition requires testing gas pressure or combustion settings, or if the code persists after you have verified wiring and sensor resistance. Gas-fired boiler diagnostics involve checking inlet gas pressure, burner flame signal, and venting, all of which require calibrated instruments and knowledge of local codes. A technician will also have access to Weil-McLain’s service bulletins and can contact the factory on your behalf if the fault is unusual or the control needs a firmware update. Do not bypass any safety limit or operate the boiler with an active fault code.
Rough cost: A pro service call runs about $150-400.