Carrier Error Code 66 — What It Means
Carrier furnace error code 66 indicates a pressure switch fault. The pressure switch monitors the negative pressure created by the inducer motor inside the heat exchanger. When the inducer starts and develops sufficient draft pressure, the pressure switch closes — signaling the IFC board that it is safe to open the gas valve. Code 66 means the pressure switch either failed to close after the inducer started, or opened unexpectedly during operation.
Code 66 differs from code 31 (pressure switch stuck open on first attempt) — code 66 specifically indicates a fault detected after the inducer is verified to be running.
Causes of Carrier Code 66
| Cause | Likelihood | Test |
|---|---|---|
| Blocked condensate trap (90%+ furnaces) | Very High | Inspect and clear trap |
| Cracked or disconnected pressure switch hose | High | Visually inspect hose |
| Failed pressure switch | Medium | Test with multimeter across switch contacts |
| Inducer motor weak / dirty wheel | Medium | Clean inducer wheel; check motor speed |
| Blocked flue or intake pipe | Medium | Inspect PVC pipes for debris or ice |
| Cracked pressure switch port on inducer | Lower | Inspect inducer housing |
Step-by-Step Diagnosis
Step 1: Check the condensate trap (90%+ furnaces only) On high-efficiency (90%+) furnaces, condensation forms in the secondary heat exchanger and drains through a condensate trap before exiting through the drain line. If this trap becomes clogged with slime or debris, it creates back-pressure that prevents the pressure switch from sensing proper draft. Remove the trap, clean it thoroughly, and reinstall. This is the most common cause of code 66 on high-efficiency Carrier furnaces.
Step 2: Inspect the pressure switch hose The rubber hose (sometimes called the pressure switch tube) connects the inducer housing port to the pressure switch input. Inspect it for cracks, kinks, or disconnections. A cracked hose will allow air to leak in, preventing proper pressure sensing. These hoses cost under $5 and are easy to replace.
Step 3: Test the pressure switch With the furnace powered on and the inducer running, use a multimeter in continuity mode across the pressure switch terminals. The switch should close (show continuity) within 30 seconds of the inducer starting. If the switch doesn’t close with the inducer running normally, the switch itself may have failed. Replacement pressure switches are available at HVAC supply houses for $15–40.
Step 4: Check the flue and intake pipes On 90%+ furnaces, the PVC flue and intake pipes must be free of obstructions. Birds nests, ice build-up in winter, and debris can block the pipes. Inspect both pipes at their termination points outside the building.
Step 5: Clean the inducer wheel Lint, dust, and debris can accumulate on the inducer wheel blades, reducing airflow and the negative pressure the inducer can develop. Remove the inducer assembly cover and inspect the wheel. A dirty wheel often reduces draft pressure just enough to prevent the pressure switch from closing reliably.
When to Call a Pro
If clearing the condensate trap and inspecting the hose don’t resolve code 66, a technician should measure the actual pressure at the switch port using a manometer to determine whether the inducer is developing adequate draft. Flue system modifications or inducer motor replacement require licensed HVAC service.