Trane Furnace Error Code 4 GREEN / 1 AMBER

This code means the furnace control board is detecting a Y signal (cooling or compressor demand) from the thermostat but is not receiving a G signal (blower or fan demand) at the same time. These two signals are supposed to arrive together during a cooling cycle. The fault indicates a wiring issue, thermostat problem, or control circuit fault that is preventing the G signal from reaching the board.

Resetting the furnace by cycling power may temporarily clear the fault display, but the code will return as soon as the thermostat sends another Y call without the G signal. The underlying wiring or thermostat issue must be corrected to resolve the fault permanently. Resetting without repairing the cause does not fix the problem.

Repair costs depend on the cause. If the fix is simply reattaching or replacing a thermostat wire, costs are typically in the range of $75 to $150 for a service call and labor. If the thermostat requires replacement, parts and labor costs generally range from $150 to $400 depending on the thermostat model. A control board replacement, if needed, can range from $300 to $600 or more including parts and labor.

Start by inspecting the G wire connections at both the thermostat and the furnace control board terminals to make sure the wire is secure and undamaged. Use a multimeter to check for 24 volts at the G terminal on the control board while the thermostat is calling for cooling. If voltage is absent at the board, the fault is in the wiring or the thermostat. A licensed HVAC technician can perform this diagnostic quickly and safely.

This fault is not a combustion or heat-related safety issue, but it does indicate an abnormal control signal condition that can prevent your cooling system from operating correctly. Running the system with this fault present may cause the compressor to operate without proper indoor airflow, which can stress cooling components over time. It is best to have the fault diagnosed and repaired promptly.