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ComfortNet data line check - Goodman *MVC95/*CVC9

Goodman/Amana *MVC95/*CVC9 two-stage ComfortNet

What is going on

How to check the two communication wires (data 1 and data 2) when the furnace, thermostat and outdoor unit are not talking.

Likely causes

  • Dip switches set wrong
  • Data wires reversed, broken or shorted
  • Wire run too long

Check in this order

  1. With power on, measure data 1 to C about 2.6 V DC. Data 2 to C: about 2.3 V DC. Data 1 to data 2: about 0.2-0.6 V DC.
  2. With all bias and termination dip switches ON you should read over 2.5 V (1-C), under 2.5 V (2-C) and over 0.2 V (1-2).
  3. On the furnace control, BIAS switches 1 and 2 and TERM switch 3 should all be ON. The outdoor unit TERM switches should also be ON.
  4. Reading 5 V / 0 V / 5 V or 0 / 0 / 0 points to a switch set wrong or a wiring fault. Fix the switch or wiring and retest.
  5. Use 18 AWG thermostat wire, max 100 ft from furnace to outdoor unit or to the thermostat.
  6. Press the learn button for about 2 seconds to reset the network after any change.

Applies to: GMVC95/AMVC95 upflow and *CVC9 counterflow two-stage furnaces

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Source: Goodman *CVC9/95 & *MVC95 Installation Instructions IO-341D (08/2010), Network Troubleshooting p.46-47; ComfortNet Wiring p.43. Written in our own words; check the manual for your exact model.

For licensed technicians. Gas, electrical and refrigerant work must be done by a qualified, licensed person. Shut off power and gas before servicing, never bypass a safety control, and follow the manufacturer’s manual for the exact model; it and local codes always take priority. HVACopedia is not affiliated with any manufacturer; brand and model names only identify the equipment.