Goodman GMVC960803BN Error Code EC4: Equipment Relay Module Missing
What Does Code EC4 Mean?
The Goodman GMVC960803BN can use an external equipment relay module on the ComfortNet network. An equipment relay module is the device that switches major system equipment — for example a paired air conditioner or heat pump — under the control of the communicating system. Code EC4 means the furnace control was configured to work with such a module but cannot find it on the network.
That can happen for a few reasons: the equipment relay module was never installed, it has come disconnected, it has lost power, or it has failed. In each case the control simply reports that the expected module is missing.
EC4 is low severity because it does not stop the furnace from heating. The furnace's own burners and blower continue to respond to heat calls as usual. What is affected is the major equipment that the missing relay module was supposed to switch — for instance, cooling from a paired AC or heat pump may not operate through the communicating system while EC4 is active.
It is important to distinguish EC4 from EC5. EC4 is about the EQUIPMENT relay module, which handles major heating/cooling equipment such as an AC or heat pump. EC5 is about the ACCESSORY relay module, which handles accessories such as a humidifier or ventilator. Both report a missing external relay module, but they govern very different loads. EC4 also differs from EC1, EC2, and EC3, which involve the thermostat link, the E-Module, and the Bluetooth module respectively.
What You'll Notice
- Paired major equipment such as an air conditioner or heat pump does not run when it should, even though the furnace heats normally
- EC4 appears on the furnace's 7-segment LED display
- The ComfortNet thermostat may indicate that an expected equipment module is missing or not responding
- Cooling or other equipment functions that route through the relay module are unavailable
- Furnace heating itself continues to work without interruption
Common Causes
| Cause | Likelihood | DIY? |
|---|---|---|
| Equipment relay module not installed or disconnected | Most common | ✗ Call a pro → |
| Faulty equipment relay module | Common | ✗ Call a pro → |
How This Is Diagnosed
A technician first determines whether an equipment relay module should be present for this installation. If one is expected, they confirm the module is physically installed, has power, and is connected to the ComfortNet network, and they inspect the wiring for loose or damaged connections. If the module is present and wired correctly but still not detected, the module itself may be faulty; if no module was ever installed, the system configuration is corrected so it 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:
- Paired equipment such as an air conditioner or heat pump will not operate while EC4 is present
- An equipment relay module is installed but is not detected after its connections have been checked — the module may need replacement
- EC4 appears but no equipment relay module was ever installed, indicating a configuration that needs correcting
- The wiring or power connection to the equipment relay module shows visible damage or looseness
Frequently Asked Questions
What does the equipment relay module do?
It switches major system equipment — such as a paired air conditioner or heat pump — under the control of the communicating ComfortNet system. EC4 means the furnace control expected this module but can't detect it.
How is EC4 different from EC5?
EC4 is the missing EQUIPMENT relay module, which controls major equipment like an AC or heat pump. EC5 is the missing ACCESSORY relay module, which controls accessories like a humidifier or ventilator. Different modules, different loads.
Will my furnace still heat with EC4 showing?
Yes. EC4 concerns an external relay module, not the furnace's heating. The burners and blower keep working; it's the major equipment tied to that module that may not operate.
✓ Verified against manufacturer service manual — March 2026