Weil-McLain A99 Error Code — What It Means
A99 does not appear as an official fault code in published Weil-McLain documentation for their boiler control platforms. Weil-McLain uses model-specific fault codes that vary by boiler family and control type. What appears on your display may be a partial reading, a custom installer code, or a display error.
Because Weil-McLain directs technicians to retrieve the stored fault history from the control and then diagnose according to the applicable service manual for that specific model, you need to first identify your exact boiler model number and control board version before any repair work begins. The correct workflow is to access the control’s diagnostic menu, pull the lockout or error log, and cross-reference the actual fault number with your model’s service documentation.
Common Causes
- Misread or partial display (~30%) The display may show only part of a longer code, or the control is in a menu mode that does not show the full fault designation.
- Non-standard or installer-programmed code (~25%) Some controls allow custom parameter labels that a technician may have set during commissioning, which do not match factory fault lists.
- Control board firmware or display fault (~20%) A corrupted memory or failing display can produce alphanumeric sequences that are not valid Weil-McLain codes.
- Wrong service manual or model mix-up (~15%) The boiler may be a different model family than assumed, so the fault table being consulted does not match the installed control.
- Recent control replacement with incorrect programming (~10%) If the control was swapped without proper setup, default parameters or test codes may appear that are not active faults.
Quick Diagnosis
Answer these to narrow it down fast.
Does the boiler's rating plate show an exact model number and control board part number?
No: Take a photo of the entire rating plate and display, then call Weil-McLain technical support or a qualified service technician to identify the unit.
Can you enter the control's diagnostic menu and see a list of past faults or lockouts?
No: The control may require a specific key sequence or installer password. Consult the manual or call a technician who can access the full diagnostic log.
Is the boiler currently running or locked out?
No: A lockout usually displays a recognized fault code. If A99 persists during lockout, the control board itself may need replacement or reprogramming.
Step-by-Step Fix
- Record the full boiler model number from the rating plate, along with the control board part number and any revision markings.
- Access the control’s diagnostic menu by following the button sequence in your service manual, and retrieve the complete fault history or lockout log.
- Cross-reference every code in the fault history with the table in your model-specific service manual or wiring diagram.
- Photograph the display showing A99 and any surrounding text or symbols, then contact Weil-McLain technical support or an authorized service distributor with the model and photo.
- Do not replace parts based on A99 alone, because the code is not documented and may not indicate a genuine component failure.
- If the boiler is under warranty, document the CP number, install date, and any failed part descriptions before ordering replacements, as Weil-McLain requires this information for warranty claims.
- Have a qualified technician verify the control board firmware version and reprogram or replace the board if it is producing invalid codes.
Parts Often Needed
| Part | Notes |
|---|---|
| Control board (model-specific) | Amazon | Only if diagnostics confirm the board is corrupted or displaying invalid codes; match exact part number from rating plate. |
| Display module | Amazon | If the LCD or LED panel is damaged and showing garbled output; verify compatibility with your boiler model. |
When to Call a Pro
Call a licensed heating technician immediately if you cannot identify your boiler model, cannot access the diagnostic menu, or if the boiler is locked out and will not restart. Gas-fired boilers require proper combustion analysis and safety checks that only qualified professionals can perform. If A99 appears alongside other symptoms such as no heat, unusual noises, or gas odors, shut off the boiler and gas supply and call for service right away. Weil-McLain also recommends that any warranty or complex control work be handled by an authorized service provider who can document the CP number, failed part number, and proper replacement procedure.
Rough cost: A pro service call runs about $150-400.