Carrier Error Code 179: Outdoor Unit Communication Fault (Meaning, Causes, Fixes)

What the code means

Carrier's communicating systems (Infinity, Evolution and related Bryant models) connect the thermostat, indoor unit, outdoor unit and accessories over a four-wire connection called the ABCD bus. Per Carrier's Infinity System Control installation instructions, A (green) and B (yellow) carry data, while C (white, 24 VAC common) and D (red, 24 VAC hot) supply power, and every ABCD connector in the system must be wired the same way. Carrier's Residential Fault Code List (Technical Information TIC2025-0010) lists code 179 as a wall control fault: "Outdoor Communication Fault." Carrier's Infinity/Evolution Troubleshooting Guide describes the same fault (shown as ODU COMM FAULT on original controls and OUTDOOR UNIT COMMUNICATION FAULT on -A models) as: the user interface has lost communication with the outdoor unit; check the wiring between the user interface and outdoor unit, check bus voltages at both, and check for a low-voltage short in the outdoor unit's control wiring. Its neighbor, 178, is the same fault for the indoor unit (furnace or fan coil). If 179 shows up during setup, the control instead says "Outdoor unit not found."

Likely causes

• A loose, swapped, corroded or broken wire on the ABCD terminals at the outdoor unit, indoor unit or thermostat. Carrier's troubleshooting guide says mis-wiring in the outdoor unit causes loss of communication.
• Damaged wire on the run to the outdoor unit, such as cable cut by a string trimmer, chewed by animals, or crushed where it enters the unit. The outdoor run is the most exposed part of the bus.
• A low-voltage short in the outdoor unit's control wiring, which Carrier names directly. Its guide also notes that a short in the outdoor wiring can block communication with the indoor unit too.
• General HVAC background, not from Carrier's 179 entry: the outdoor disconnect pulled or loose, or the outdoor breaker tripped, so the outdoor unit's control is off.
• Electrical noise from wire run close to high-voltage or other electrical wiring. Carrier says to separate the wires and, if needed, use shielded thermostat wire for random, intermittent communication errors.
• A failed outdoor control board, which should only be suspected after the wiring and bus voltages check out.

Safe DIY checks

1. Write down every code the thermostat shows, including any 178 (indoor) or 180-182 (zoning and network interface) codes. Several communication codes at once point toward a wire or board pulling down the whole bus.
2. Check the outdoor disconnect box next to the outdoor unit (make sure the pull-out or switch is fully in and on) and the outdoor unit's breaker in your panel. Reset a tripped breaker once; if it trips again, leave it off and call a pro.
3. Walk the visible thermostat wire from the house to the outdoor unit and look for cuts, chew marks, crushed spots or bare copper. Don't splice or tape it yourself; note what you find for the technician.
4. Cycle power: turn off the breakers for the indoor and outdoor units for about a minute, then turn the indoor unit back on first and the outdoor unit after.
5. With power off, pull the thermostat off its base and check that the four ABCD wires are seated and tight, with no bare copper touching another terminal. Don't change which wire goes where, and leave the outdoor unit's electrical panel closed.
6. If 179 comes back, or the thermostat shows "Outdoor unit not found," call a Carrier dealer. Testing the bus and the outdoor control wiring means working inside a unit with line voltage.

Parts that commonly fix it

Always match the part number to your exact model.

Related codes

• 178 Carrier HVAC System 178
• 196 Carrier HVAC System 196
• 44 Carrier HVAC System 44
• 82 Carrier HVAC System 82
• 180.02 Trane HVAC System 180.02

All Carrier codes →

Sources

1. Carrier Technical Information TIC2025-0010, Residential Fault Code List (communicating systems): 178 Indoor Unit Communication Fault; 179 Wall Control, Outdoor Communication Fault; 180-182 zone and NIM communication faults
2. Carrier Infinity/Evolution Troubleshooting Guide, Rev. 2: Outdoor unit communication fault (ODU COMM FAULT / OUTDOOR UNIT COMMUNICATION FAULT), check wiring, bus voltages and low-voltage shorts in outdoor wiring; isolating faults to the outdoor unit; intermittent noise and shielded wire (distributor-hosted copy)
3. Carrier Infinity System Control Installation Instructions (SYSTXCCITC-07SI): ABCD bus colors and functions, consistent wiring, 18 AWG over 25 ft, "Outdoor unit not found," bus isolation tip