Weil-McLain A105 Error Code — What It Means
A105 does not appear in Weil-McLain manufacturer documentation as a boiler fault code. The code A105 is documented for Rheem/Ruud hybrid water heaters, where it signals an evaporator temperature sensor failure. If you see A105 on a Weil-McLain boiler display, verify the exact model and control platform, then consult the owner’s manual or wiring diagram for that unit. The fault code families Weil-McLain documents include temperature sensor or thermistor faults, low-water conditions, circulator or pump problems, and control-related issues, but the precise code depends on the specific boiler series.
Without manufacturer confirmation that A105 applies to your Weil-McLain model, treat it as a generic sensor or wiring fault. Common underlying problems are failed temperature sensors, loose or corroded sensor connectors, open or shorted sensor circuits, or control-board sensing errors. Always confirm the appliance brand and model before ordering parts, because the same code can mean different things on different equipment.
Before You Replace Anything
Homeowners sometimes replace the control board when the real fault is a failed sensor or corroded connector. Inspect wiring and measure sensor resistance against the manufacturer’s chart before ordering any board.
Common Causes
- Wrong appliance or code mismatch (~40%) A105 is documented for Rheem/Ruud hybrid water heaters, not Weil-McLain boilers, so the displayed code may be misread or the unit may not be a Weil-McLain.
- Failed temperature sensor or thermistor (~30%) A damaged or out-of-specification sensor will send incorrect readings or no signal to the control board.
- Loose, corroded, or damaged sensor wiring (~20%) Broken wires or oxidized connector pins create open or high-resistance circuits that mimic sensor failure.
- Control-board sensing circuit fault (~10%) If the sensor and wiring both test good, the input circuit on the board itself may be defective.
Quick Diagnosis
Answer these to narrow it down fast.
Does your owner's manual list A105 as a valid code for your boiler model?
No: The code may be from a different appliance or a misread display; confirm the boiler brand and model before proceeding.
Can you locate the temperature sensor wiring and visually inspect the connectors for corrosion or damage?
No: Call a licensed HVAC technician to access the sensor and control board safely.
Does the boiler show any additional fault codes or warning lights along with A105?
No: Focus troubleshooting on sensor and wiring integrity for the component listed in your manual.
Step-by-Step Fix
- Confirm appliance identity. Verify that the unit is a Weil-McLain boiler and record the exact model and serial number from the data plate.
- Consult the owner’s manual. Look up A105 in the fault-code table for your specific model; if it does not appear, contact Weil-McLain or a technician.
- Power down the boiler. Turn off electrical power at the service switch and the circuit breaker to prevent shock during inspection.
- Inspect sensor wiring and connectors. Locate the temperature sensor listed in the manual, check for loose pins, corrosion, or broken wires, and clean or reseat connections as needed.
- Measure sensor resistance. Unplug the sensor from the board, use a multimeter set to ohms, and compare the reading to the manufacturer’s resistance chart for the current ambient temperature.
- Replace the sensor if out of specification. Install a new sensor or thermistor that matches the part number in the manual, then reconnect the wiring securely.
- Restore power and monitor. Turn the breaker and service switch back on, clear the fault if the control allows, and observe whether the code returns during normal operation.
Parts Often Needed
| Part | Notes |
|---|---|
| Temperature sensor or thermistor (model-specific) | Amazon | Match the part number in your boiler’s service manual; resistance values vary by design. |
| Sensor wiring harness or connector | Amazon | Use if the existing connector is corroded or the wire insulation is damaged. |
When to Call a Pro
Call a licensed HVAC or boiler technician if the owner’s manual does not list A105, if you cannot safely access the sensor or control board, or if the fault persists after replacing the sensor and verifying all wiring. Gas-fired and high-voltage boiler work requires proper training, combustion testing, and code compliance. A technician will use specialized diagnostic tools to measure sensor output, check control-board inputs, and confirm safe operation of the boiler and its safety interlocks before returning the system to service.
Rough cost: A pro service call runs about $150-350.