Goodman GMVC960803BN Error Code EC5: Accessory Relay Module Missing
What Does Code EC5 Mean?
The Goodman GMVC960803BN can use an external accessory relay module on the ComfortNet network. An accessory relay module is the device that switches comfort accessories — for example a whole-home humidifier, a ventilator (ERV/HRV), or an electronic air cleaner — under the control of the communicating system. Code EC5 means the furnace control was set up to work with such a module but cannot find it on the network.
As with the other missing-module alarms, this usually means the accessory relay module was never installed, has come disconnected, has lost power, or has failed. The control responds by reporting that the expected accessory module is absent.
EC5 is low severity and does not affect the furnace's ability to heat. Burners and blower continue to run normally on demand. What is affected are the accessories the module was meant to switch — a humidifier may not run, a ventilator may not cycle, or an electronic air cleaner may not energize while EC5 is active.
The key contrast is with EC4. EC5 is about the ACCESSORY relay module, which handles comfort accessories like a humidifier or ventilator. EC4 is about the EQUIPMENT relay module, which handles major equipment such as an air conditioner or heat pump. Both codes report a missing external relay module, but they control different kinds of devices. EC5 is also separate from EC1 (the thermostat data link), EC2 (the E-Module), and EC3 (the Bluetooth module).
What You'll Notice
- Accessories such as a humidifier, ventilator, or electronic air cleaner do not run when they should, even though the furnace heats normally
- EC5 appears on the furnace's 7-segment LED display
- The ComfortNet thermostat may indicate that an expected accessory module is missing or not responding
- Humidity or ventilation controls tied to the module have no effect
- Furnace heating continues without interruption
Common Causes
| Cause | Likelihood | DIY? |
|---|---|---|
| Accessory relay module not installed or disconnected | Most common | ✗ Call a pro → |
| Faulty accessory relay module | Common | ✗ Call a pro → |
How This Is Diagnosed
A technician first determines whether an accessory relay module should be present for this installation. If one is expected, they confirm the module is physically installed, powered, and connected to the ComfortNet network, and inspect the wiring for loose or damaged connections. If the module is present and correctly wired but still undetected, the module may be faulty; if no module was ever installed, the configuration is corrected so the system no longer expects one.
When to Call a Professional
This code involves components that are not homeowner-serviceable, so have a licensed HVAC technician diagnose and repair it. Keep in mind:
- A humidifier, ventilator, or air cleaner will not operate while EC5 is present
- An accessory relay module is installed but is not detected after its connections have been checked — the module may need replacement
- EC5 appears but no accessory relay module was ever installed, indicating a configuration that needs correcting
- The wiring or power connection to the accessory relay module shows visible damage or looseness
Frequently Asked Questions
What does the accessory relay module do?
It switches comfort accessories — such as a humidifier, ventilator (ERV/HRV), or electronic air cleaner — under the control of the communicating ComfortNet system. EC5 means the furnace control expected this module but can't detect it.
How is EC5 different from EC4?
EC5 is the missing ACCESSORY relay module, which controls accessories like a humidifier or ventilator. EC4 is the missing EQUIPMENT relay module, which controls major equipment like an AC or heat pump. They handle different devices.
Does EC5 stop my furnace from heating?
No. EC5 concerns an external accessory relay module, not the furnace's core heating. The furnace keeps running; only the accessories tied to that module are affected.
✓ Verified against manufacturer service manual — March 2026