g/hvac • Posted by u/hvac_hero • 23 hours ago

Carrier VRF E9 fault — outdoor unit randomly dropping IDU communication

Carrier 38MGQB36 VRF outdoor unit, 8 indoor units, 14 months in service. Started getting E9 communication faults 3 weeks ago. Random IDUs drop, no pattern to which unit.

Communication wiring: 3-wire shielded twisted pair (A, B, GND), 350 feet total trunk, daisy-chained. Shield grounded at outdoor unit only per Carrier spec.

Already replaced the OAT sensor and the comms board in the outdoor unit (warranty). Issue persists. Bus noise reading is 15mV — normally closer to 5mV.

Have I got an EMI problem or something in the wiring?

💬 4 replies

4 replies

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u/duct_doctor • 23 hours ago

Also worth checking: any broken shield continuity at the splice points. A floating shield section on a long run can act as an antenna and actually make interference worse. Check shield continuity end-to-end before assuming the source is external.

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u/minisplit_mike • 23 hours ago

If you can't find the EMI source, add a 120-ohm termination resistor at the far end of the RS-485 bus. Carrier's field tech docs mention this for runs over 300 feet. Can't hurt to try before opening walls.

u/freon_frank • 23 hours ago

Had this exact problem on a Daikin VRV install. Turned out the comm cable was routed through the mechanical room near a 7.5HP exhaust fan VFD. Added a ferrite clamp on the comm cable where it entered the mech room. Noise went from 18mV to 4mV. Problem solved, never came back.

u/admin_jake ADMIN • 23 hours ago

15mV on the RS-485 bus is your problem. That's 3x the normal noise floor. At 350 feet, you're pushing the edge of what RS-485 can handle cleanly, and any EMI source in the building can tip it over. First question: is any section of that comm cable running parallel to high-voltage wiring or near a VFD?