Weil-McLain Boiler A123 Error — What It Means
A123 does not appear in documented Weil-McLain fault code lists. Weil-McLain boilers use model-specific error codes, and the control display shows a number or letter sequence that must be cross-referenced against the fault table printed in that model’s installation and service manual. The same symptom can be reported differently depending on the control platform (AquaBalance, EcoNet, or other). If you saw A123 in a service history or contractor menu, it may be a stored past fault rather than the active lockout condition.
Before proceeding, find the boiler nameplate and write down the exact model number, size, and serial number. Then locate the fault-code table in the manual for that model. If the display is currently showing A123, take a clear photo of the screen and the boiler nameplate. This information is essential for accurate diagnosis and for any warranty or parts claim, since Weil-McLain requires the model, size, serial number, and failed part description before processing service requests.
Before You Replace Anything
Without the correct fault meaning, technicians may replace sensors, igniters, or gas valves at random. Always pull the model-specific fault table and check the diagnostics menu for the active fault history before ordering parts.
Common Causes
- Code not in manufacturer documentation (~50%) A123 does not match any published Weil-McLain fault code, so it may be a display error, a code from a different brand, or a stored fault from the diagnostics menu.
- Model-specific code not yet referenced (~30%) The code exists for your particular control platform but requires the exact model manual to decode, since Weil-McLain fault lists vary by boiler series.
- Contractor menu history code (~15%) The display is showing a past fault from the service history log rather than the current active lockout, and you need to exit the menu to see the live status.
- Misread or transposed code (~5%) The actual fault on screen is A12, A23, or another code that was misread or recorded incorrectly.
Quick Diagnosis
Answer these to narrow it down fast.
Does your boiler's installation manual include a fault-code table?
No: Note your exact model number and download the manual from Weil-McLain's website or call their technical support with the model and serial number.
Is the display currently flashing or locked out, or is the boiler running normally?
No: If the boiler runs normally, A123 may be a stored history code. Exit the contractor menu to see the live status, or power-cycle the boiler and note the code that appears next.
Have you accessed the contractor or diagnostics menu on the control?
No: The code you see is likely the active fault. Confirm the model number and look it up in the fault table before replacing any parts.
Step-by-Step Fix
- Locate the boiler nameplate and write down the complete model number, size (input BTU), and serial or CP number.
- Take a clear photograph of the control display showing the A123 code, including any other indicators, LEDs, or status messages on the screen.
- Find the installation and service manual for your exact model by searching Weil-McLain’s website or calling technical support with the model number.
- Open the fault-code table in the manual and search for A123. If the code is not listed, check whether the display is in the contractor or diagnostics menu rather than showing the live fault.
- Exit any service menus by pressing and holding the appropriate button sequence (typically documented on the control or in the manual) to return to the normal status screen.
- Record the active fault code that appears after exiting the menu or after a power cycle, then cross-reference that code in the model’s fault table.
- Follow the model-specific troubleshooting steps in the manual for the verified fault code, checking sensor resistance, gas pressure, vent condition, or other parameters as directed.
Parts Often Needed
| Part | Notes |
|---|---|
| Model-specific sensor or control module | Amazon | Part identity depends entirely on the true fault code and boiler model. Do not order until the code is verified. |
| Installation and service manual (download) | Amazon | Free from Weil-McLain’s website when you enter your exact model number. |
When to Call a Pro
Call a licensed heating technician immediately if you cannot locate the boiler model number, if the manual’s fault table does not list the code you see, or if the troubleshooting steps require checking gas pressure, testing ignition components, or measuring sensor resistance. Weil-McLain boilers integrate gas controls, pressure switches, and flame-proving circuits that must be diagnosed with a multimeter and combustion analyzer. A technician will retrieve the fault history from the control, verify the active fault against the model-specific table, test the sensors and switches in the fault path, and replace only the components that fail specification. Attempting to replace parts without decoding the correct fault wastes money and can leave the root cause unresolved.
Rough cost: A pro service call runs about $150-400.