Weil-McLain A104 Error Code — What It Means
An A104 error code does not appear in available Weil-McLain boiler documentation or manufacturer service materials. Weil-McLain boilers use different fault-code schemes that vary by control platform and model. The exact meaning of any error code on your boiler depends on the specific control board installed and must be verified against your unit’s manual and CP number. If you see A104 on a Rheem electric water heater, it means upper tank temperature sensor failure, but that definition does not transfer to Weil-McLain hydronic boilers.
Before diagnosing further, locate your boiler’s nameplate and CP number, then pull the fault history from the diagnostic or contractor menu if your control supports it. Weil-McLain controls store past faults and display them in service mode. Match the displayed code to your exact model’s manual rather than assuming a generic meaning. Most boiler lockouts stem from sensor circuit faults, wiring damage, or system operating conditions like low pressure or circulation problems. Do not replace parts until you confirm the actual fault code and what it represents on your specific boiler.
Before You Replace Anything
Homeowners sometimes replace sensors or control boards without first checking the fault history and wiring. Pull the diagnostic menu and inspect all sensor connectors and harnesses for loose pins, corrosion, or damage before ordering any part.
Common Causes
- Unverified or misread code (~40%) The displayed code may be a different sequence or the manual was not consulted for the exact control platform installed on your boiler.
- Sensor or wiring fault (~30%) Open or shorted temperature sensor circuit, damaged harness, loose connector, or failed thermistor can trigger boiler lockouts on Weil-McLain units.
- System operating condition (~20%) Low water pressure, circulation problems, or closed valves can cause control lockouts that must be cleared after correcting the system issue.
- Control board input fault (~10%) If sensor and wiring test correctly but the fault persists, the control board may have a failed input requiring board replacement.
Quick Diagnosis
Answer these to narrow it down fast.
Does your boiler control display a service or contractor menu with fault history?
No: Locate your boiler's installation and service manual using the model number and CP number from the nameplate, then look up the code definition for your specific control.
Are any sensor connectors loose, corroded, or visibly damaged?
No: Measure the suspected sensor's resistance at room temperature if you have the manufacturer's ohm-chart for that exact sensor, or call a technician to test the circuit.
Is your boiler's pressure gauge reading below 12 psi or showing a low-pressure condition?
No: The fault is likely sensor or control-related rather than a system pressure issue, so focus diagnostic work on wiring and sensor circuits.
Step-by-Step Fix
- Turn off power to the boiler at the service switch or circuit breaker and confirm the display is dark.
- Locate the nameplate on the boiler jacket or inside the burner door and write down the full model number, serial number, and CP number.
- Find the correct manual for your model by searching the Weil-McLain website or calling technical support with your CP number, since code definitions are control-specific.
- Access the diagnostic menu if your control has one by following the button sequence in your manual, then scroll through and record all stored fault codes.
- Inspect all sensor connectors at the boiler water sensor, flue sensor, and any aquastat or limit controls for loose pins, corrosion, or unplugged harnesses.
- Test the suspected sensor with a multimeter set to ohms if you have the resistance-versus-temperature table for that exact sensor, or replace it if the manual points to a sensor fault.
- Clear the fault per your manual’s reset procedure, restore power, and verify normal operation by monitoring the display and checking that the burner fires on a call for heat.
Parts Often Needed
| Part | Notes |
|---|---|
| Boiler water temperature sensor / thermistor | Amazon | Match the part number in your boiler’s manual or wiring diagram, sensors are model-specific. |
| Sensor wiring harness | Amazon | Order the correct harness if pins are damaged or the wire insulation is melted or cracked. |
| Control board / display module | Amazon | Only replace if sensor and wiring test correctly but the fault persists and the manual confirms a control-side failure. |
When to Call a Pro
Call a licensed boiler technician if you cannot find an A104 definition in your Weil-McLain manual, if the fault history shows codes you do not recognize, or if you are uncomfortable working with 120V control circuits and gas appliances. A technician can access the service menu, retrieve the real fault code, test sensor circuits with the correct resistance tables, and verify system operating conditions like pressure and circulation. Professional diagnosis is especially important if the boiler repeatedly locks out after you clear the fault, if you smell gas, or if the control board shows physical damage. Many boiler lockouts turn out to be simple wiring or sensor issues, but confirming the exact code and testing the correct circuit prevents expensive misdiagnosis and ensures safe operation.
Rough cost: A pro service call runs about $200-450.