Carrier Error Code 21: Gas Heating Lockout
LED Blink Pattern
2 short blinks, 1 long blink.
How to Fix Carrier Code 21
- Step 1: Turn off the furnace at the breaker box to prevent further gas accumulation.
- Step 2: Ensure all gas appliances are off and the gas meter is shut off.
- Step 3: Immediately call a licensed HVAC technician and your gas company to report a possible gas leak.
How to Reset Carrier Code 21
- Turn off the furnace power switch, usually located on or near the unit.
- Turn off the circuit breaker for the furnace. Wait 30 seconds.
- Restore power at the breaker and flip the furnace switch back on.
- Set the thermostat to call for heat. If the error code returns after 2-3 ignition attempts, the fault requires repair.
Resetting clears the code from the display but does not fix the underlying fault. If the code returns after reset, proceed to the diagnostic steps above.
What Does Carrier Error Code 21 Mean?
The Carrier 21 error code, also known as a gas heating lockout, signals that your furnace has tried to ignite the gas multiple times but failed. After several unsuccessful attempts, the system locks itself down as a safety precaution. This is often due to a problem in the ignition sequence or a detected hazard. It's important to address this issue promptly to restore heating and ensure your safety. Continuing to run a system with a lockout can potentially lead to dangerous conditions or further damage to the unit. Contact a qualified HVAC technician to diagnose and repair the problem.
Common Causes
- Failed or weak igniter (hot surface igniter or spark igniter)
- Faulty gas valve not opening to allow gas flow
- Flame sensor not detecting the flame, even when present
- Blocked or restricted flue causing incomplete combustion
- Low gas pressure from the gas supply
Diagnostic Steps
| If... | Check | Expected |
|---|---|---|
| Furnace attempts ignition but fails | Measure igniter resistance with a multimeter | Typically 50-200 ohms (check manufacturer specs) |
| Igniter resistance is within range | Measure voltage to the gas valve during ignition | 24VAC |
| Gas valve voltage is present | Inspect the flame sensor for proper placement and cleanliness | Sensor should be clean and properly positioned in the flame |
| Flame sensor is clean and positioned correctly | Measure flame sensor microamps (uA) during burner operation | Typically 2-10 uA (check manufacturer specs) |
Applicable Carrier Models
- 24ABC6 Air Conditioner(electric)
- 25HBC5 Heat Pump(electric)
- Base Series(gas)
- Base Series Furnace(gas)
- Comfort 80 Gas Furnace(gas)
- Comfort Series(electric)
- Comfort Series Furnace(gas)
- Comfort Series Heat Pump(electric)
- Infinity 20 Heat Pump with Greenspeed Intelligence(electric)
- Infinity 80(gas)
- Infinity Series(electric)
- Infinity Series Furnace(gas)
- Infinity Series Heat Pump(electric)
- Performance Boost 95 Furnace(gas)
- Performance Boost Furnace(gas)
- Performance Series(gas)
- Performance Series Furnace(gas)
- Performance Series Heat Pump(electric)
- WeatherMaker 8000(gas)
- WeatherMaker 9300(gas)
- WeatherMaker Furnace(gas)
Replacement Parts
| Part | OEM Number | Est. Cost | Where to Buy |
|---|---|---|---|
| Hot Surface Igniter | — | — | — |
| Gas Valve | — | — | — |
| Flame Sensor | — | — | — |
Related Codes on Same Models
Other codes that affect the same Carrier model families as 21.
| Code | Description | Severity |
|---|---|---|
| 12 | Blower On After Power Up | Warning |
| 13 | Limit/Rollout Fault | Critical |
| 14 | Ignition Lockout | Critical |
| 22 | Abnormal Flame | Critical |
| 23 | Pressure Switch Fault | Warning |
| 24 | Secondary Voltage Fuse | Warning |
| 31 | High Stage Pressure Fault | Warning |
| 32 | Low Flame Sense | Warning |
| 33 | Limit/Rollout Switch Lockout | Critical |
| 34 | Ignition/Flame Sense Fault | Warning |