What This Error Means

A five-blink code indicates a twinning-circuit fault on furnace controls that support twinned operation. Rheem notes that thermostat wiring or an electronic thermostat can sometimes contribute to this code even when two furnaces are not intentionally twinned.

Symptoms You May Notice

Furnace may fail to respond normally to a heat call; two connected furnaces may not operate together; intermittent fault; code may appear after thermostat installation or wiring changes.

Common Causes

Incorrect wire connected to the twin terminal; mismatched integrated furnace controls in a twinned system; incorrect control-board switch setting; thermostat or thermostat-wiring fault; loose connection; control-board failure.

Safe Troubleshooting Steps

1. Turn off furnace power. 2. Confirm whether the installation actually uses two twinned furnaces. 3. Inspect accessible thermostat wiring for an unintended connection to the twin terminal. 4. Do not move control-board wires unless the wiring diagram is available and the installer is qualified. 5. A qualified technician should confirm board compatibility, switch settings, thermostat wiring, and twinning connections and isolate the thermostat circuit as described in Rheem service guidance.

Parts Commonly Involved

Integrated furnace control; twin terminal and wiring; thermostat; thermostat cable; board configuration switches.

When to Call a Technician

Yes. Twinning and control-board diagnosis should be performed by an HVAC technician familiar with Rheem wiring and configuration.

Safety Notice

Disconnect furnace power before inspecting control wiring. Incorrect connections can damage the control board or cause unsafe equipment operation.

Manufacturer Documentation