Weil-McLain Boiler A31 Error Code — What It Means
An A31 code does not appear in published Weil-McLain boiler manuals or fault-code tables. The display may have been read incorrectly, or the code shown is specific to a third-party control that is not documented in standard Weil-McLain literature. Weil-McLain boilers use model-specific fault codes, and technicians are directed to retrieve the stored fault history through the control’s diagnostics menu rather than relying on a single alphanumeric display. To find the correct meaning, locate your boiler’s exact model number (usually on the rating plate), enter the control’s fault-history or diagnostics mode following the instructions in your installation and operating manual, and note the full fault code or event name shown. That code can then be matched to the troubleshooting table in your model’s manual for the manufacturer’s corrective action, required component checks, and any spec limits.
Before You Replace Anything
Homeowners often replace the flame sensor or igniter when the real issue is an incorrect code readout or a control-display fault. Always retrieve the actual fault history from the control menu and verify the code against your model’s manual before ordering parts.
Common Causes
- Misread display or non-standard control (~40%) The characters shown may be a segment-display artifact, a third-party controller code, or a partial code that requires the full fault history to interpret.
- Control board or display fault (~25%) A failing control module or corrupted display can show codes that do not exist in the manufacturer’s published list.
- Incorrect manual or cross-reference (~20%) Using a manual for a different Weil-McLain series or a generic boiler guide can lead to code confusion.
- Stored legacy fault (~15%) Older or updated controls may retain fault codes from a previous software version or a replaced module.
Quick Diagnosis
Answer these to narrow it down fast.
Does your boiler display show the code continuously, or does it appear only in the fault-history menu?
No: If it appears only in stored history, the condition may have cleared. Review all stored faults in sequence and address the most recent or most frequent one first.
Can you find your boiler's model number and locate the matching installation manual online or in your paperwork?
No: Without the correct manual you cannot verify what the code means. Contact Weil-McLain customer service or a qualified technician who can pull the correct documentation for your unit.
Is the boiler completely locked out and refusing to fire, or is it running normally despite the code?
No: If the boiler runs normally, the code may be a stored event from a transient condition. Still, log the full fault history and monitor for recurrence before deciding no service is needed.
Step-by-Step Fix
- Power off the boiler at the service switch or circuit breaker, wait thirty seconds, then restore power to reset the control and clear any temporary display glitch.
- Locate the boiler rating plate on the jacket or inside the front panel and write down the complete model number, serial number, and control type (for example, a Honeywell or Weil-McLain-branded module).
- Access the control’s diagnostics menu by pressing and holding the button sequence described in your installation manual, typically a combination of mode and up/down buttons held for several seconds until the display enters service mode.
- Record every stored fault code shown in the history, including the fault number, any alphanumeric code, and the timestamp or cycle count if displayed.
- Match each recorded code to the fault table in your model’s manual. If A31 does not appear, note that discrepancy and prepare to share the full fault list with a technician.
- Check the control’s firmware version if your manual describes how to view it. Older firmware may display codes differently than current documentation.
- Call a licensed boiler technician with your model number, control type, and the complete fault-history printout so they can bring the correct manual, parts, and diagnostic tools on the first visit.
Parts Often Needed
| Part | Notes |
|---|---|
| Boiler control board or module | Amazon | Only if the technician confirms the control itself is faulty or shows corrupt codes not in the manual. |
| Flame sensor (UV or ionization rod) | Amazon | Order only after the tech retrieves the actual fault and the manual directs a flame-sensing check. |
When to Call a Pro
Call a licensed boiler technician immediately if the boiler is locked out, if you cannot find the correct manual for your model, or if the fault-history menu shows codes you cannot match to any published Weil-McLain table. Gas-fired boilers involve combustion safety, gas pressure, venting, and control logic that require proper test instruments and manufacturer training. A qualified technician will retrieve the full fault history, confirm which control is installed, consult the model-specific manual or factory support, and perform the manufacturer’s prescribed corrective action. Attempting to bypass a lockout or replace parts based on an unverified code can create unsafe operating conditions and void your warranty.
Rough cost: A pro service call runs about $150–350 for a diagnostic visit and typical sensor or control repair.