Error Code 21
Carrier 59SC5A Error Code 21: Gas Heating Lockout
TL;DR
Control will NOT auto reset. Check for mis-wired gas valve or defective control (valve relay). Disclaimer
This article is for informational purposes only and does not guarantee the accuracy or completeness of the information provided. Always turn off power and gas supply before attempting any repairs. If you smell gas, leave immediately and call your gas company. Consult a licensed HVAC technician for diagnosis and repair. Any actions taken based on this information are at your own risk. What Does Code 21 Mean?
Control will NOT auto reset. Check for mis-wired gas valve or defective control (valve relay).
Common Causes
| Cause | Likelihood | DIY? |
|---|---|---|
| Mis-wired gas valve | Common | ✗ Call a pro → |
| Defective control board (valve relay) | Common | ✗ Call a pro → |
When to Call a Professional
Contact a licensed HVAC technician if:
- Basic troubleshooting did not resolve the error
- You smell gas at any point during the process
- You are not comfortable working around gas lines or electrical components
- The unit needs a gas valve, control board, or motor replacement