Carrier 59SC2B Error Code 45: Control Circuitry Lockout
What Does Code 45 Mean?
Status code 45 on the Carrier 59SC2B is a control circuitry lockout that auto-resets after one hour. The lockout is triggered by one of three internal control board failures: a gas valve relay stuck in the open position, a flame sense circuit failure, or a software check error.
A stuck-open gas valve relay means the board cannot send voltage to the gas valve, preventing ignition. A flame sense circuit failure means the board's internal circuitry for reading the flame sensor signal has malfunctioned, making it impossible to confirm whether the burner is lit. A software check error indicates the board's microprocessor detected an internal consistency problem.
As a first step, reset power to the furnace by turning off the breaker for 30 seconds. If the code was caused by a transient software glitch, the furnace may return to normal after the reset. However, if code 45 reappears — especially within a few heating cycles — the control board has a permanent hardware fault and must be replaced. Do not continue to reset and operate a furnace that repeatedly shows this code.
Common Causes
| Cause | Likelihood | DIY? |
|---|---|---|
| Gas valve relay stuck open on control board | Common | ✗ Call a pro → |
| Flame sense circuit failure | Common | ✗ Call a pro → |
| Software check error on control board | Uncommon | ✗ Call a pro → |
When to Call a Professional
Contact a licensed HVAC technician if:
- If the code returns after a power reset — the control board needs replacement
- If the furnace fails to operate normally after resetting
✓ Verified against manufacturer service manual — March 2026