Weil-McLain Boiler A180/E18 Error — What It Means
The A180 code (also displayed as E18 on some Weil-McLain boilers) indicates that the outlet or supply water temperature has exceeded 210°F. The control board has detected an overtemperature condition on the water leaving the boiler and locks out to protect the appliance from damage.
This fault does not mean the boiler has actually overheated in every case. A failed or drifting temperature sensor, loose wiring, or poor water circulation can all trigger the same high-limit alarm even when the water is at a safe temperature.
Before You Replace Anything
Homeowners often replace the control board first. Before ordering any board, re-seat the sensor connectors and swap the supply and return sensors if they are identical; if the fault moves with the sensor, the sensor is bad and the board is fine.
Common Causes
- Failed or drifting supply/outlet temperature sensor (~45%) The sensor that monitors water leaving the boiler reads incorrectly or has drifted out of calibration, signaling a false high-temperature alarm to the control board.
- Loose, corroded, or poorly seated sensor wiring (~25%) Connectors at the sensor or control board become intermittent, causing the controller to misread the temperature signal and fault on a phantom overheat.
- Circulator pump not running or running weakly (~15%) When the pump fails to move water through the boiler, heat builds up in the heat exchanger and the outlet temperature climbs rapidly above the 210°F limit.
- Air in the system or closed isolation valves (~10%) Trapped air pockets or a closed valve block circulation, preventing heat from leaving the boiler and pushing the outlet temperature past the high limit.
- Control board misreading sensor input (~5%) The board itself develops a fault in the temperature-input circuit and reads a normal sensor signal as an overtemperature condition.
Quick Diagnosis
Answer these to narrow it down fast.
Does the circulator pump run when the boiler calls for heat?
No: No flow explains the overheat. Troubleshoot or replace the circulator pump first.
After re-seating connectors, does the fault clear and stay off?
No: Suspect the sensor itself. Proceed to swap or replace the supply sensor.
If you swap supply and return sensors (when identical), does the fault move to the other position?
No: Sensor is likely good. Inspect the control board or call a technician for advanced diagnostics.
Step-by-Step Fix
- De-energize the boiler by switching off the circuit breaker or removing the fuse, then wait one minute before opening the control panel.
- Inspect and re-seat all sensor connectors at both the supply/outlet sensor and the control board, looking for corrosion, bent pins, or loose contacts.
- Verify the circulator pump runs during a call for heat and that the system is properly filled with no air binding or closed valves blocking flow.
- Swap the supply and return temperature sensors if your model uses matching sensors, then power up and observe which position faults; if the code moves with the sensor, replace that sensor.
- Replace the supply/outlet temperature sensor if the fault follows the sensor or if resistance and actual water temperature do not match expected behavior.
- Check system flow conditions by opening any closed isolation valves, bleeding air from high points, and confirming heat can leave the boiler into the distribution piping.
- Inspect the control board for damaged solder joints or corrosion around the sensor-input terminals if the fault persists after sensor and circulation checks, or call a qualified technician for board-level diagnostics.
Parts Often Needed
| Part | Notes |
|---|---|
| Supply/outlet temperature sensor | Amazon | Match the sensor type and thread size to your boiler model; some use NTC thermistor probes. |
| Return temperature sensor | Amazon | Used for diagnostic swapping if identical to the supply sensor; also stock as a spare. |
| Sensor wiring harness or connector kit | Amazon | If connectors are corroded or damaged; confirm pin count and connector style before ordering. |
| Circulator pump | Amazon | Replace if the pump does not run or runs but delivers no flow; verify voltage and flange size. |
When to Call a Pro
Call a licensed boiler technician if you are not comfortable working with line voltage, if the fault persists after re-seating connectors and verifying pump operation, or if you lack a multimeter to test sensor resistance. A technician can compare sensor readings against actual water temperature, perform control-board diagnostics, and check for combustion or limit-switch issues that may contribute to real overheating. Gas-fired boiler work often requires a licensed contractor in most jurisdictions, and attempting board-level repair without proper training can void warranties or create safety hazards.
Rough cost: A pro service call runs about $200-400.