On a Daikin Rebel rooftop unit with the factory A2L leak detection system, the Refrigerant Leak alarm trips when at least one sensor detects refrigerant above 15% of the refrigerant Lower Flammability Level (LFL). It stays on until it is manually cleared. A separate Refrigerant Sensor Fault alarm trips when a connected sensor fails or the leak board loses communication. Both are described in Daikin OM 1407, MicroTech Unit Controller for Rebel Refrigeration Only Controls (August 2026).
What triggers the Refrigerant Leak alarm?
OM 1407 (pp. 16-17) says the leak detection control board triggers a leak alarm when at least one sensor detects a refrigerant concentration above 15% of the refrigerant LFL. The board passes its status to the MicroTech unit controller over Modbus (p. 16). The sensors, if there is more than one, are wired in a daisy chain and terminated at the mitigation board (p. 16).
What is the difference between the leak alarm and the sensor fault alarm?
| Alarm | What trips it (OM 1407) | How it clears |
|---|---|---|
| Refrigerant Leak | At least one sensor detects refrigerant above 15% of LFL (p. 17) | Stays on until manually cleared (p. 17) |
| Refrigerant Sensor Fault | A connected sensor is determined to be faulty, such as a self-test failure or loss of communications (p. 16). The manual says a fault can be caused by a sensor malfunctioning or being disconnected, or by a loss of Modbus communication between the MicroTech unit controller and the A2L board (p. 18) | Stays on until manually cleared (p. 18) |
The manual also says the sensors have self-reporting diagnostics that the mitigation board monitors, and that the board triggers a Fault alarm if a sensor fails (p. 18). A digital output closes a normally open contact if a leak or a faulty sensor is detected (p. 8), and two alarm relays are provided for field connection (p. 16).
What does the unit do when a leak is detected?
OM 1407 describes the response for a unit that is enabled and for a unit that is disabled (pp. 17-18).
| Item | Unit enabled | Unit disabled |
|---|---|---|
| Compressors | Deactivated and locked out | Remain locked out |
| Supply fan | Supply fan minimum speed controls are overridden so the fan does not run below a predetermined speed | Supply fan is turned on, with the same minimum speed override |
| Outside air damper | On a DOAS unit without a return air opening, the field outside air damper signal is ignored and the damper goes to 100% | Stays closed, except on a DOAS unit without a return air path, where it opens to 100% |
| VAV box output | Not listed | The VAV box digital output opens, to open the boxes |
For the disabled case the manual lists exceptions in which MicroTech will not activate the mitigation steps: an open E-Stop circuit (E-Stop takes priority over the A2L leak alarm) and high discharge or return air temperature alarms (above 170°F) being triggered (p. 18). The manual says that after no refrigerant has been detected for five minutes, an enabled unit can resume normal operation and a disabled unit reverts to its previous disabled operation (pp. 17-18), but the leak alarm stays active and the refrigeration system stays locked out until the alarm is manually cleared (p. 17).
How do you clear the Refrigerant Leak alarm?
OM 1407 says the alarm "will remain on until manually cleared" but does not give keypad or menu steps for the leak or sensor fault alarms. The manual does document an Alarm Reset input (p. 10): when it changes from OFF to ON, the current active alarms are cleared, and it must go OFF and back ON to issue another reset. The manual does not say this input is the way to clear the leak alarm, so confirm the method for your unit with Daikin Applied or your installer.
Our practical advice: find out why the alarm tripped before clearing it, because a repeat alarm means the sensor still sees refrigerant or the sensor chain is faulty.
How do you diagnose the leak board and sensors?
OM 1407 (p. 19) describes a display and push button on the Leak Detection Control Board:
| Button hold time | What the board does (p. 19) |
|---|---|
| Less than 2 seconds | Shows the last 10 sensor faults (loss of communication or a faulted state reported by a specific sensor) |
| 2 to 5 seconds | Shows sensor status, including the current LFL level and any loss of communication or fault |
| 5 to 10 seconds | Starts a mitigation test of about 5 minutes; the board goes into alarm mode and the MicroTech controller begins the mitigation sequence |
| More than 10 seconds | Shows all the GID values supported by the sensor board |
At power-up the board display shows which sensors are detected: SX = 1 means sensor X is active and communicating, and SX = 0 means it is not communicating or inactive (p. 19). A general configuration fault (Flt CFG) is shown when the expected number of sensors does not match. The "Number of sensors configured" parameter ranges from 1 to 8 with a default of 2 (Table 1, p. 20). If a sensor shows SX = 0 or Flt CFG, check the daisy chain wiring and the configured sensor count. That check is our advice, not a step in the manual.
Do the refrigerant sensors need to be replaced on a schedule?
OM 1407 (p. 18) says the sensors are not considered "Limited Life Sensors" and, under normal operation, are not expected to be replaced within the life expectancy of the unit. It does not state a number of years, and it does not state where the sensors are mounted.
Parts involved
| Component | Role in this issue | Where to find it |
|---|---|---|
| A2L leak detection control board | Evaluates sensor readings, triggers the leak and sensor fault alarms and reports to the MicroTech controller over Modbus. | Request a quote with the unit model and serial number |
| Refrigerant leak sensor(s) | Detect refrigerant and report self-diagnostics to the board. A faulty or disconnected sensor causes the Sensor Fault alarm. | Request a quote with the unit model and serial number |
| MicroTech unit controller | Runs the mitigation sequence and receives the leak status over Modbus. | Request a quote with the unit model and serial number |
We do not currently list a Rebel leak sensor or leak detection board online, so all three items above are quote-only.
Scope: This article covers the refrigerant leak detection and mitigation described in Daikin OM 1407 (August 2026) for the MicroTech unit controller on Rebel refrigeration-only controls. OM 1407 lists R-32 as the refrigerant and does not mention R-454B, so do not assume it applies to other Rebel configurations. Other Daikin controllers use different indications. Parts mentioned are examples of what to look up, not a fit confirmation.
Before you order
Read the unit model and serial number from the nameplate. See also Daikin Applied, McQuay, and Daikin-McQuay Part Numbers: Same Board, Three Names, Daikin Water Source Heat Pump MicroTech III LED Codes: What Each Light Means and Daikin Applied Replacement Parts: OEM Parts for Chillers, Air Handlers & Commercial HVAC. Browse Daikin Parts for Daikin Applied. To ask about a leak sensor or board, use our quote request form or contact us at help@totalhvacparts.com or 302-219-4994 with the unit model, serial number and the alarm shown.
Frequently Asked Questions
What triggers the refrigerant leak alarm on a Daikin Rebel unit?
Per Daikin OM 1407, the leak detection control board triggers the alarm when at least one sensor detects refrigerant above 15% of the refrigerant Lower Flammability Level (LFL).
Does the Daikin Rebel refrigerant leak alarm reset itself?
No. OM 1407 says the Refrigerant Leak alarm stays on until manually cleared, and the refrigeration system stays locked out until then. After five minutes with no refrigerant detected, an enabled unit can resume normal operation (a disabled unit reverts to its previous disabled operation), but the alarm still needs a manual clear. The manual also documents an Alarm Reset input that clears active alarms.
What does a Refrigerant Sensor Fault alarm mean on a Daikin Rebel?
OM 1407 says it is triggered when a connected sensor is faulty, for example a self-test failure or loss of communications. A sensor malfunctioning or disconnected, or a loss of Modbus communication between the MicroTech controller and the A2L board, can cause it. It also stays on until manually cleared.
What does a Daikin Rebel unit do when it detects a refrigerant leak?
Per OM 1407, compressors are deactivated or kept locked out, the supply fan minimum speed controls are overridden, and on a DOAS unit without a return air path the outside air damper opens to 100%. The manual also lists a VAV box output that opens when the unit is disabled.
Sources
- Daikin OM 1407, MicroTech Unit Controller for Rebel Refrigeration Only Controls (A2L refrigerant leak detection, pp. 16-18; Alarm Reset input, p. 10; leak board diagnostics and Table 1, pp. 19-21)
- Live product titles for the parts listed above, as shown on each product page
A2L refrigerants such as R-32 are mildly flammable. Rooftop units involve line voltage, pressurized refrigerant and rotating equipment. If a refrigerant leak is suspected, service should be performed only by qualified personnel, following the manufacturer's manual for your unit and your facility's safety and lockout/tagout procedures.