Carrier 59SC5A Error Code 45: Control Circuitry Lockout
What Does Code 45 Mean?
Status code 45 on the Carrier 59SC5A is a control circuitry lockout triggered by an internal failure on the HK42FZ036 control board. The lockout auto-resets after one hour, and three possible internal failures can cause it: a gas valve relay stuck open, a flame sense circuit failure, or a software check error.
When the gas valve relay is stuck open, the board physically cannot send voltage to the gas valve, so ignition is impossible regardless of other conditions. A flame sense circuit failure means the board's internal electronics for reading the flame sensor microamp signal have malfunctioned, preventing the board from confirming whether the burner is lit. A software check error indicates the board's processor detected an internal inconsistency.
Try resetting furnace power by turning off the breaker for 30 seconds. A transient software error may clear with a power cycle. If code 45 returns after the reset — either immediately or within subsequent heating cycles — the control board has a hardware-level failure. Continuing to reset and operate the furnace is not recommended; the board should be replaced by a qualified technician.
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 reappears after a power reset — the control board must be replaced
- If the furnace fails to operate after power cycling
✓ Verified against manufacturer service manual — March 2026