Rheem Furnace Code 66: Blower Cutback, Static Pressure Too High

What the code means

In Rheem's fault code table for its 80% two-stage variable-speed furnaces (802V installation instructions 92-24161-149-06, Table 14), code 66 is "Blower Cutback," a non-critical fault. The variable-speed blower motor is already running at the highest rpm or torque its specifications allow, but it would need more to deliver the desired airflow against the current static pressure (the resistance of the filter, coil, ducts and registers). The motor's software keeps it from going past that limit and sends the fault to the furnace control. Rheem says all operation, including thermostat calls, continues as normal, though blower operation may be slightly compromised. The code only shows during the first three minutes of blower operation after a power reset, isn't logged to the fault history after the first hour, and is logged at most once. Rheem describes it as a tool to tell the installer about inadequate airflow from excessive duct static pressure, "not intended to be a fault code" but an operating indicator. The R96V condensing furnace manual describes the same code as "Overspeed Operation": the motor is being forced into its speed limit, usually by restrictive or confined ductwork or many closed supply registers.

Likely causes

• A dirty or clogged air filter, or a filter that's too restrictive for the system (Rheem lists the filter needing cleaning or replacing as a cause)
• Many or all supply registers closed, or registers and return grilles blocked by furniture or rugs (Rheem R96V manual)
• Ductwork that's undersized, poorly designed or restricted, so static pressure is above the furnace's published limit (Rheem 802V manual)
• Something else in the air path restricting flow, such as a dirty evaporator coil or a collapsed or crushed duct
• On R96V furnaces, a blower wheel set screw that isn't tight on the motor shaft, which Rheem says can cause erratic motor operation and code 66

Safe DIY checks

1. Remember that code 66 is a warning. The furnace keeps running, so there's no emergency, but the airflow problem is worth fixing because low airflow makes the system work harder.
2. Check the air filter and replace it if it's dirty. Make sure it's the right size and installed with the airflow arrow pointing toward the furnace.
3. Walk the house and open every supply register. Clear furniture, rugs and boxes away from registers and return grilles.
4. Look at any ducts you can see in the basement, attic or crawl space for crushed, kinked or disconnected sections.
5. Turn the furnace power off and back on, then watch the display. Rheem says 66 only appears during the first three minutes of blower operation after a power reset, so if it doesn't come back after you've fixed the filter and registers, the restriction may be solved.
6. If the code keeps returning with a clean filter and open registers, have an HVAC technician measure static pressure. Duct changes are a job for a pro.

Parts that commonly fix it

Always match the part number to your exact model.

Related codes

• 0 Rheem Gas Furnace 0
• 11 Rheem Gas Furnace 11
• 30 Rheem Gas Furnace 30
• 57 Rheem Gas Furnace 57
• 58 Rheem Gas Furnace 58

All Rheem codes →

Sources

1. Rheem 802V downflow installation instructions 92-24161-149-06, Table 14 fault codes: 66 Blower Cutback (non-critical, motor at maximum rpm or torque for the static pressure, shown only in the first three minutes after power reset, logged once, an operating indicator rather than a fault; solution: fix ductwork or restriction or clean or replace the filter so static pressure doesn't exceed published values)
2. Rheem R96V installation instructions 92-24161-124-01, Diagnosing Blower Motor Issues: Overspeed Operation (code 66) from restrictive or confined ductwork or closed supply registers, and loose blower wheel set screw causing erratic operation and code 66