Weil-McLain A16 Error Code — What It Means
The A-16 error code on a Weil-McLain AquaBalance Central Heating Only boiler indicates the outdoor temperature sensor is not connected or installed. The control module detects an open circuit at the outdoor sensor input terminals during startup and halts the heating call until the fault is cleared. If an outdoor sensor is not physically present and has not been exempted in the system parameters, the boiler will display A-16 and refuse to fire. A faulty sensor that gives extreme readings or a short circuit typically triggers a different fault (F-39), so A-16 specifically points to a missing or disconnected sensor.
Before You Replace Anything
Homeowners sometimes replace the control module thinking the board is bad, when the real issue is simply a missing sensor or loose wiring at the outdoor reset terminals. Measure resistance across the sensor wires (should be 8,000-15,000 ohms) before buying any parts.
Common Causes
- No sensor installed (~40%) The outdoor temperature sensor is physically missing, and the system has not been set to exempt the sensor via the parameter menu.
- Loose or disconnected wiring (~30%) Sensor wires are not properly seated in the control module terminals or have come loose during service.
- Wrong terminal connection (~15%) The sensor wires are plugged into incorrect terminals (such as DHW instead of Outdoor Reset), creating an open circuit at the expected input.
- Fault not cleared after repair (~10%) The sensor was installed or reconnected but the A-16 fault was not manually reset, so the code persists.
- Damaged wiring harness (~5%) The sensor cable is chafed, broken, or has an internal open circuit between the sensor head and the boiler.
Quick Diagnosis
Answer these to narrow it down fast.
Is an outdoor temperature sensor physically installed on the exterior of the building?
No: Decide if you need outdoor reset control. If not, exempt the sensor by holding ECO + DHW+ for 10 seconds, navigating to the outdoor sensor parameter, and changing the value to 1. If you do need it, install the sensor and connect it to the correct terminals.
After confirming or exempting the sensor, did you press and hold the RESET button for 1.5 seconds?
No: The fault will not clear on its own. Press and hold RESET for 1.5 seconds, then restart the heating call.
Does measuring resistance across the sensor wires at the boiler give 8,000-15,000 ohms?
No: If you read 0 ohms or infinite resistance, the sensor is bad or the wiring is broken. Replace the sensor or repair the cable.
Step-by-Step Fix
- Verify sensor connection. Confirm the outdoor temperature sensor is physically installed and its wires are plugged into the correct Outdoor Reset terminals on the control module, not DHW or other contacts.
- Measure resistance at the boiler. Disconnect the sensor wires at the boiler terminals and use a multimeter to measure resistance across the two leads. You should see 8,000-15,000 ohms depending on outdoor temperature. If you read 0 ohms or open circuit, the sensor is faulty or the wiring is broken.
- Inspect wiring integrity. Trace the sensor cable from the outdoor unit to the boiler, looking for chafing, pinched insulation, loose connections at the sensor head, or breaks in the wire.
- Exempt the sensor (if not needed). If your installation does not require outdoor reset control, press and hold ECO + DHW+ buttons for 10 seconds to enter the parameter menu. Navigate to the outdoor sensor parameter (default 0) and change it to 1 using the minus button, then exit the menu.
- Clear the fault. After confirming the sensor connection or exempting it, press and hold the RESET button on the boiler control for 1.5 seconds to clear the A-16 code.
- Restart the system. Call for heat from the thermostat and verify the boiler fires normally without displaying A-16. If the code returns, recheck wiring and connections.
- Replace the sensor or harness. If resistance is out of range and wiring is intact, replace the outdoor temperature sensor. If the cable is damaged, replace the wiring harness between the sensor and the boiler.
Parts Often Needed
| Part | Notes |
|---|---|
| Weil-McLain AquaBalance outdoor temperature sensor kit | Amazon | Confirm the exact part number for your boiler model from the installation manual or serial tag. |
| Outdoor sensor wiring harness | Amazon | Only needed if the cable is cut, chafed, or shows an open circuit between the sensor and the boiler. |
When to Call a Pro
Call a qualified boiler technician if you are not comfortable working with low-voltage control wiring or entering the boiler’s parameter menu. A technician can quickly verify sensor resistance, trace wiring faults, and configure the system to exempt the sensor if it is not required for your installation. If you have replaced the sensor and wiring and the A-16 code persists, the control module input may be damaged and requires professional diagnosis and replacement. Any work involving gas supply pressure (3.5-11 inches w.c.) or system water pressure (12-15 PSI) also requires a licensed professional.
Rough cost: A pro service call runs about $150-300.