Carrier 59TP6 Furnace Problems & Error Codes — Overview
The Carrier 59TP6 is a residential gas furnace equipped with a diagnostic LED on the control board and a self-test feature for troubleshooting. The board displays blink codes that correspond to specific fault conditions, and the most frequent field failures involve airflow restrictions, pressure-switch issues, low-voltage shorts, and flame-sensing problems. The exact meaning of each code is documented in Carrier’s published troubleshooting guide, and technicians should always match the observed blink pattern to the fault table before beginning repairs.
Common real-world issues include dirty filters driving limit-switch lockouts, pressure-tubing blockages causing pressure-switch faults, shorted low-voltage wiring blowing the secondary fuse, and contaminated flame sensors or leaking gas valves triggering abnormal flame signals. Most 59TP6 service calls can be traced to external causes like blocked return air, restricted vents, or filter neglect rather than board or valve failures. Gas-side work and valve replacement should be handled by a licensed technician.
Most Common Problems on This Model
- Error code 33 limit or rollout lockout Restricted airflow from a dirty filter, blocked supply or return registers, failed limit switch, or open rollout switch trips the safety circuit and locks out the furnace until the cause is cleared and the board is reset.
- Error code 23 pressure switch did not open Obstructed or kinked pressure tubing, a stuck-closed pressure switch, weak inducer performance, or vent restrictions prevent the pressure switch from opening and stop the heating cycle.
- Error code 24 secondary voltage fuse blown A short in the 24-volt wiring, miswired thermostat or accessory circuit, or failed low-voltage load causes overcurrent and opens the fuse on the control board.
- Error code 22 abnormal flame signal Flame sensed when the gas valve is off, caused by a leaking or stuck-open gas valve, contaminated flame rod, poor grounding, or wiring faults in the flame-sensing circuit.
- Error code 21 gas heating lockout Repeated ignition failures, bad gas-valve wiring, a failed gas valve, or control-board output issues drive the furnace into lockout after multiple unsuccessful heating attempts.
- Furnace cycles on high limit repeatedly Dirty filter, undersized return ductwork, closed registers, or blower motor running too slow cause the heat exchanger to overheat and trip the limit switch.
- Inducer motor runs but burners do not light Pressure-switch failure to close, blocked vent pipe, disconnected or damaged pressure tubing, or failed igniter prevent the control from opening the gas valve.
- Flame sensor reads inconsistent or weak signal Oxidation or carbon buildup on the flame rod reduces microamp signal to the board and causes nuisance shutdowns or ignition retries.
Parts That Commonly Fail
| Part | Notes |
|---|---|
| Control board | Amazon | Houses the diagnostic LED and manages all furnace functions, fuse, and safety circuits. |
| Pressure switch | Amazon | Proves inducer airflow and can stick closed or fail to close due to mechanical wear or debris. |
| Flame sensor (flame rod) | Amazon | Detects flame presence and commonly fails due to oxidation or contamination from combustion gases. |
| Limit switch and rollout switch | Amazon | High-temperature safety devices that open on overheating or flame spillage and may fail mechanically over time. |
When to Call a Pro
Call a licensed HVAC technician for any gas-valve diagnosis or replacement, control-board replacement, pressure-switch testing that requires combustion-analyzer verification, or if you observe repeated limit or rollout lockouts that do not resolve after clearing airflow obstructions. Any work involving gas piping, valve replacement, or combustion tuning requires a professional with proper licensing and test equipment. If the diagnostic LED shows a code you cannot identify or if the furnace locks out immediately after reset, a technician with access to the Carrier fault table and service manual is needed to trace wiring, test safety circuits, and verify proper operation of the inducer, igniter, and flame-proving system.