Lost inverter communications (system malfunction): the outdoor control (AOC) and the inverter (MOC) have stopped communicating. Fault 33 is logged after 2 minutes with no communication and is elevated to 48 after 3 consecutive failures within 20 minutes, or 20 minutes of continuous loss of connection.
Carrier 48 means the outdoor control (AOC) and the inverter (MOC) have stopped communicating. The LED flashes 4 short and 8 long. Fault 33 is the earlier local version: the manual logs 33 after 2 minutes with no communication, and elevates that to 48 after 3 consecutive failures within 20 minutes, or after 20 minutes of continuous loss. A 33 pattern is 3 short and 3 long. Causes are a loose or disconnected harness between AOC plug PL20 and the MOC, radio or electrical noise, and possible inverter damage. For noise, the manual says the system will try to self-mitigate with repeated start attempts. A technician checks the harness, then the inverter board: a steady green LD5 means the 5 volt link between AOC and MOC is intact, and a blinking amber LED3 means the MOC is communicating to the AOC. If the harness is good, the manual's action for possible inverter damage is to change the inverter drive. That work is inside a cabinet that carries line voltage.
Likely causes
- Loose or disconnected harness between the AOC (PL20) and the MOC
- Radio or electrical noise
- Possible damage to the inverter
Safe checks for 48
- Note the flash pattern on the amber status LED: 4 short flashes then 8 long flashes is 48; 3 short then 3 long is 33. Short flashes give the first digit and long flashes the second
- For radio or electrical noise, the manual says the system will try to self-mitigate with repeated start attempts
- Technicianverify a good harness connection between the AOC (PL20) and the MOC
- Technicianon the inverter's MOC board, a steady green LD5 indicates the 5 volt connection between AOC and MOC is OK, and a blinking amber LED3 indicates communication from the MOC to the AOC
- Technicianif the harness is good, the manual's action for possible inverter damage is to change out the inverter drive
Capacitors can hold a charge after the power is off. Safety terms.
When to call a pro
Call a licensed HVAC technician: fault 48 involves the harness inside the outdoor unit and possibly the inverter drive, both of which carry high voltage.
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Multimeter for appliance and HVAC diagnosis: guide
Technician step in this code: on the inverter's MOC board, a steady green LD5 indicates the 5 volt connection between AOC and MOC is OK, and a blinking amber LED3 indicates communication from the…
This one is for a technician to test or replace, so we only explain it and do not link to buy it.