How to Test an A2L Leak Sensor and Mitigation Sequence

Why this matters

On an A2L system that carries a refrigerant detection system, the sensor and the board's response to it are a safety function, not an accessory. If the sensor reads a leak, the equipment is supposed to stir the room air so a pocket of refrigerant cannot build to a flammable concentration near the floor, and to shut down anything that could ignite it. Every one of those actions depends on field wiring someone landed on install day. A sensor that is plugged in and showing a green light proves the sensor is alive; it does not prove the blower starts, the compressor stops or the heat strips drop out. This procedure proves the whole chain, on the install and on any visit where the sensor, the board or the heat kit was touched.

Which systems need a sensor, where it mounts and what the listing requires belongs to See related: A2L Equipment Install Requirements Field Card. What to do when a sensor trips on a real leak belongs to A2L Leak Detection and Mitigation Response. This card is the functional test between them.

Start state

You need the equipment maker's installation instructions for this air handler or furnace and its sensor (the test method, the board's status codes and the mitigation sequence are the maker's, and they differ), a CAT III or better meter proven on a known live source, a clamp meter, a thermometer for the supply air, and an A2L-rated leak detector.

Check before you begin:

• No real leak. Walk the indoor unit with the A2L-rated detector first. If it alarms, stop: this is no longer a test, it is a leak, and you follow A2L Leak Detection and Mitigation Response with ignition sources kept away from the area.
• The charge is in and the system runs. You are testing behavior during a cooling or heating call, so the system must already run normally on both.
• Your test method is one the maker permits. Never trip the sensor by releasing refrigerant from the system or from a cylinder. Knowingly venting refrigerant is prohibited under 40 CFR 82.154, and a burst of A2L refrigerant in an air handler closet is the hazard this sensor exists to manage. Use the board's test function, the maker's simulated-fault method (commonly unplugging the sensor, which the board reads as a sensor fault and answers with mitigation), or a test gas only where the maker names one.

The steps

1. Confirm the sensor is mounted and wired as the instructions show. With power off at the air handler disconnect, locked and tagged, and proved dead with a tester proven on a known live source before and after, per NFPA 70E Article 120 (120.5 in the 2021 edition, renumbered in 2024), check that the sensor sits where the instructions put it (usually low in the coil enclosure or drain pan area, since A2L refrigerants are heavier than air), that its harness is fully seated at the board, and that the harness is clear of the heat strips and the blower. A sensor clipped to the wrong bracket or a harness resting on a strip element passes a status check and fails in service.

2. Restore power and read the sensor status. Close the disconnect from the side of the cabinet with the blower door on, then read the board's status LED or display against the maker's code table. Accept only the code for sensor present and normal. Many sensors run a warm-up period after power-up before they report; wait it out rather than reading a warm-up code as a fault. A fault code at this step is a wiring or sensor problem to fix before going further.

3. Map the field wiring the mitigation depends on. With the blower door off and the disconnect locked off again and re-proved dead, trace each output the mitigation sequence controls against the maker's diagram. The three that matter on a heat pump with strips: the blower command, the compressor call (Y) to the outdoor unit, and the heat strip control. On most A2L-equipped air handlers the thermostat's Y and W land on the board, and the board passes them through or blocks them. If an installer has run the thermostat W straight to the heat kit, or Y straight to the condenser, the board cannot interrupt them and mitigation cannot work, though the status light stays green.

4. Start a call and run the test. Doors on, power restored, set a call: cooling for the compressor-stop check, then a heating call with strips for the heat-lockout check. Trigger the maker's test. Note the time you trigger it, because the maker's sequence has response times you are going to verify. Hazard: if your method is unplugging the sensor, do it with the disconnect off and locked, then restore power, rather than reaching into an energized board area.

5. Verify each mitigation action, one at a time. With the test active:

• Blower: the indoor blower runs at the maker's mitigation airflow even with the thermostat satisfied. Confirm by airflow at a supply register, not by sound alone.
• Compressor: the outdoor unit stops within the maker's stated time. Confirm at the condenser: contactor open, compressor amps 0 A on the clamp.
• Heat source: electric strips de-energize. Confirm with the clamp on the strip supply conductors at the air handler, reading 0 A. On a gas furnace with an A2L coil, the maker's sequence will name what it locks out; verify that the burner does not light on a heating call during the test.

The clamp readings mean working at the air handler with the strip supply conductors energized at 240 V. Do it with the clamp only, through the access the maker provides, with the meter proven on a known live source before and after and the PPE your shop's 70E risk assessment assigns, and keep fingers out of the strip compartment.

6. Clear the test and watch the recovery. End the test the maker's way (reconnect the sensor with power locked off, or end the test mode at the board). Many sequences hold the blower on for a set time after the sensor clears before returning to normal operation. Time it against the instructions. Then confirm the system returns to the call it was answering: compressor back on cooling, strips back on heat. Energy goes back into both units at this step without anyone touching a switch, so the cabinet doors are on and nobody is at the condenser before you clear the test.

7. Record the result. On the install or service record: sensor location, the status code read at step 2, the test method used, each mitigation action with its measured value (airflow present, compressor amps, strip amps) and response time, and the recovery time. A test with no numbers recorded is indistinguishable from a test that was not done.

Worked example: the strips that stayed on

A 3-ton R-454B heat pump with an air handler and a 10 kW strip kit in two 5 kW stages. The sensor mounts in the coil compartment; the maker's test is a board button held for five seconds.

Step 1. Power locked off and proved dead. Sensor on its bracket below the coil, harness seated and tied clear of the strip kit.

Step 2. Power on. Status LED cycled through warm-up, then showed the maker's sensor-normal code.

Step 3. Wiring traced against the diagram. Y from the thermostat landed on the board's Y input, and the board's Y output went to the condenser. W from the thermostat did not land on the board at all: it ran straight to the heat kit's sequencer terminal. The status light was green; the strips were outside the board's reach. The tech noted it and, rather than re-wiring on assumption, ran step 5 to prove what it would do.

Step 4 and 5, cooling call. Test triggered at 10:02. Compressor amps fell from 12.4 A to 0 A at 10:02, inside the maker's stated time, contactor open. Blower stayed on at mitigation airflow with air moving at the supply registers. Pass.

Step 4 and 5, heating call with strips. Room thermostat set 4 F above room temperature so both stages called. Before the test, strip supply read 41.2 A. Test triggered at 10:11. Compressor dropped to 0 A. Blower ran. Strip supply still read 41.2 A thirty seconds later. Fail: in a real leak the strips would have kept glowing in the same enclosure the sensor was alarming in.

That strip current fits the kit: at its 240 V rating it draws 10,000 / 240 = 41.7 A, so 41.2 A at the 242 V measured at the air handler supply is both elements on, within element tolerance.

The fix. Disconnect off, locked, proved dead. The thermostat W moved to the board's W input and the board's heat output wired to the sequencer per the maker's diagram. Doors on, power restored, a fresh heating call: strip supply 41.2 A, the board passing heat normally.

Retest. Test triggered at 10:38. Compressor 0 A, blower on, strip supply 0 A within the maker's stated time. Test cleared at 10:40; the blower held on for the maker's post-clear period and the system returned to heat with strips at 41.2 A. Recorded per step 7, including the original wiring fault and the time it was corrected.

What changes the answer

• The board answers a sensor fault with a lockout, not just mitigation. Some boards hold the system off until a person resets it. Then step 6 includes the reset, and the record says so, because the customer will otherwise call a lockout a breakdown.
• A gas furnace with an A2L coil. The heat source is a burner, not strips. Which outputs the furnace control interrupts, and whether a separate coil-mounted board does it, is the maker's design; verify at the burner, not at the wiring.
• A ducted system on a zoning panel. The zone dampers decide whether mitigation airflow reaches the room the equipment sits in. The maker or the zoning panel's instructions will say whether dampers drive open on a leak signal; verify it rather than assuming.

How to verify you got it right

The record carries a measured value for every mitigation action: compressor amps 0 A, strip amps 0 A, blower running with airflow at the registers, each with its time from trigger. The sensor-normal code is written down. Every output the sequence controls runs through the board, which you traced, not inferred from a green light. And the system went back to normal operation after the test cleared, which you watched happen before you closed the cabinet.

References

• 40 CFR 82.154, prohibition on knowingly venting refrigerant
• NFPA 70E, Article 120 (120.5 in the 2021 edition, renumbered in 2024), current edition as your safety program adopts it
• Equipment maker's installation instructions for the sensor, the board's status codes and the mitigation sequence
• See related: A2L Equipment Install Requirements Field Card
• See related: A2L Leak Detection and Mitigation Response