Carrier Furnace Error Code 43

Code 43 on a Carrier furnace means the low-heat pressure switch, draft safeguard switch, or auxiliary limit switch is reading open at the same time the high-heat pressure switch is reading closed. This contradiction in pressure switch states indicates a problem with draft sensing, pressure tubing, or one of the switches themselves. The furnace shuts down as a safety precaution until the conflict is resolved.

Yes, code 43 is a safety-related lockout. The pressure switches that trigger this code are designed to confirm that proper combustion air and exhaust draft are present before and during burner operation. When the control board detects a conflict between these switch states, it stops the furnace to prevent incomplete combustion or unsafe flue gas conditions. Do not attempt to bypass the pressure switches.

Repair costs depend on the root cause. Reconnecting or replacing pressure tubing is generally inexpensive, with parts typically costing a few dollars to around $15 to $30. A replacement pressure switch generally ranges from $20 to $75 or more depending on the specific switch. Adding in labor, a service call to diagnose and repair a code 43 fault typically falls in the range of $100 to $350, depending on the technician's rates and the time required for diagnosis.

Some causes of code 43 are DIY-accessible, such as reconnecting a loose pressure tube or clearing a blocked pressure port. However, testing pressure switches accurately requires a manometer, and diagnosing an auxiliary limit or draft safeguard switch often requires understanding of the furnace wiring schematic. If basic tubing inspections do not resolve the fault, a qualified HVAC technician should complete the diagnosis.

Resetting the furnace by cycling the power may temporarily clear the lockout and allow the furnace to attempt another ignition sequence, but the code will return if the underlying problem is not corrected. A reset is useful as a first step to see if the fault was a one-time anomaly, but recurring code 43 faults require proper diagnosis of the pressure switch circuit, tubing, and any installed auxiliary switches.