Weil-McLain A151 Error — What It Means
Weil-McLain does not publish an A151 error code in its official diagnostic documentation. If the number 151 appears on your boiler’s display, it is most likely showing the current water temperature (151°F), which is a normal operating temperature for hydronic heating systems and not a fault. If the boiler is locked out and showing an error message alongside 151, the real issue is probably an Internal Fault code from the control module or a failed thermistor (temperature probe) causing false high-temperature readings. Check the contractor diagnostics menu (hold up and down arrows, then navigate to Fault History and Past Errors) to confirm the actual error code, because codes like Internal Fault, E02, or A02 are real documented faults and A151 is not.
Before You Replace Anything
Homeowners often replace the control module or ignition parts when the real cause is a failed thermistor in the low-water cutoff sensor. Test the thermistor’s resistance with a multimeter before replacing any expensive components.
Common Causes
- Failed thermistor (temperature probe) (~40%) The thermistor built into the low-water cutoff sensor sends false high-temperature readings, causing the boiler to think it is overheating and lock out even though the actual temperature is normal.
- Control module internal error (~25%) A hardware fault or power surge inside the control board causes an Internal Fault code that stops the boiler from running.
- Low system water pressure (~15%) Water pressure below 12 psi triggers multiple safety faults including internal errors, and the boiler will not fire until pressure is restored and leaks are fixed.
- Dirty or misaligned flame sensor (~10%) A corroded or soot-covered flame sensor cannot detect the burner flame reliably, causing the boiler to lock out after ignition attempts fail.
- Blocked vent or intake (~10%) Birds’ nests, ice, or condensation in the flue or intake pipe restrict airflow and trigger safety lockouts.
Quick Diagnosis
Answer these to narrow it down fast.
Does the display show exactly 151 with no error message or lockout light?
No: The boiler is locked out with a real fault code. Check the contractor diagnostics menu to find the actual error and follow the steps below.
Is the system water pressure below 12 psi on the gauge?
No: Pressure is okay. The problem is likely a failed thermistor, dirty flame sensor, or control module fault, and you need a licensed technician to diagnose and repair it.
Can you see any ice, debris, or birds' nests blocking the vent or intake pipes?
No: The blockage is not visible or does not exist. The fault is internal to the boiler and requires a technician to test sensors and the control module.
Step-by-Step Fix
- Access the contractor diagnostics menu by holding the up and down arrow buttons together until the menu appears, then navigate to Fault History and Past Errors to confirm the actual error code (not A151).
- Check system water pressure on the gauge and verify it is above 12 psi. If low, open the filling loop valve to add water until the gauge reads about 15 psi, then close the valve and check for leaks.
- Inspect the vent and intake pipes for blockages like ice, birds’ nests, or condensation. Clear any obstructions and make sure both pipes are open and intact.
- Power cycle the boiler by turning it off at the disconnect switch, waiting 30 seconds, then turning it back on to clear transient faults and attempt a restart.
- Call a licensed HVAC technician if the boiler remains locked out. The technician will test the thermistor resistance with a multimeter, clean or replace the flame sensor, check gas flow, and replace the control module if needed.
- Do not attempt repairs yourself on gas connections, combustion sensors, or the control module, because Weil-McLain requires licensed gas fitters for warranty and safety reasons.
Parts Often Needed
| Part | Notes |
|---|---|
| Thermistor (temperature probe) for Weil-McLain boiler | Amazon | Match your exact model number; often part of the low-water cutoff assembly |
| Flame sensor for Weil-McLain boiler | Amazon | Confirm compatibility with your control board and model |
When to Call a Pro
Call a licensed HVAC technician immediately if the boiler is locked out and you cannot find a simple cause like low water pressure or a blocked vent. Gas-fired boilers require a licensed gas fitter to work on combustion components, sensors, and control modules, and Weil-McLain warranty claims require professional diagnosis with the CP number, failed part number, and photos. Do not attempt to replace the thermistor, flame sensor, or control board yourself unless you have gas-fitting certification and the proper test equipment, because incorrect work can create carbon monoxide hazards and void your warranty.
Rough cost: A pro service call runs about $200-400.