Carrier rooftop flame sensor
Use for codes 11, 12, 14, or 34 after cleaning the rod and checking grounding.
$15-$70
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Carrier Rooftop Unit Error Codes: Complete Technician Guide
Carrier 48/50 Series rooftop units (RTUs) communicate faults via a diagnostic LED on the integrated furnace control (IFC) board. Each flash count maps to a specific fault code. Newer units with 7-segment displays show codes directly. This guide covers all common Carrier RTU fault codes.
How to Read Carrier RTU Flash Codes
Count rapid LED flashes, wait for the 3-second pause, then count again. The number equals the fault code. Most boards also store the last 5 fault codes — press and hold the diagnostic button for 5 seconds to retrieve fault history.
Carrier RTU Flash Code Table
| Flash Code | Fault Description | Common Cause |
|---|---|---|
| 2 | Low-pressure lockout | Low refrigerant, dirty filter, blocked coil |
| 3 | High-pressure lockout | Dirty condenser coil, failed condenser fan, overcharge |
| 4 | Open high-pressure switch | Refrigerant overcharge, blocked condenser |
| 11 | Ignition failure | No gas, weak spark, dirty flame sensor |
| 12 | Flame sense fault | Dirty/cracked flame sensor, grounding issue |
| 13 | Limit switch open | Restricted airflow, dirty filter, failed blower |
| 14 | Ignition lockout | 3 failed ignition attempts — requires manual reset |
| 21 | Gas valve fault | Failed gas valve, wiring issue |
| 22 | Low combustion air | Blocked flue, failed inducer |
| 23 | Draft safeguard switch open | Blocked flue, failed inducer motor |
| 24 | Secondary voltage fuse blown | Short in low-voltage wiring |
| 25 | Control reversing valve fault | Stuck reversing valve (heat pump models) |
| 31 | High-pressure switch open | Refrigerant overcharge, condenser blocked |
| 32 | Low-pressure switch open | Low refrigerant, metering device issue |
| 33 | Limit switch lockout | Persistent overtemperature — check all airflow paths |
| 34 | Ignition proving fault | Flame sensor issue, gas pressure low |
| 41 | Blower motor fault | Failed blower motor or run capacitor |
| 42 | Inducer motor fault | Failed inducer, blocked flue |
| 43 | Low-ambient lockout | Ambient temp below unit minimum rating |
| 44 | Control board fault | Replace integrated control board |
| 45 | Control board memory fault | Replace control board |
| 46 | Power failure | Power interruption logged |
| 48 | Induced draft motor lockout | Failed inducer motor or blocked flue |
| 52 | Defrost fault | Failed defrost board or sensor issue |
| 54 | Defrost thermostat open | Check defrost thermostat and wiring |
| 58 | Compressor fault | Compressor locked out — check contactor and capacitor |
Most Common Carrier RTU Faults
Code 13 — Limit Switch Open
The most common commercial RTU service call. Always check in order:
- Air filter — replace if dirty
- All supply/return grilles — confirm open
- Blower wheel — inspect for debris buildup
- Limit switch continuity — open at room temp means failed switch
Code 23 — Draft Safeguard Switch
Inducer-related fault. Check inducer motor amp draw and measure flue pressure before condemning the pressure switch.
Code 33 — Limit Switch Lockout
Three consecutive limit trips trigger a hard lockout. Root cause is almost always restricted airflow. Fix the airflow restriction before resetting.
Codes 11/14 — Ignition Issues
Check gas pressure first: 3.5 in. w.c. for natural gas, 10 in. w.c. for propane. Clean or replace the flame sensor rod — measure microamp output (must be above 1.5 µA).
Code 3/31 — High Pressure
Wash condenser coil with coil cleaner. Verify all condenser fans are rotating. Check refrigerant subcooling: target 10–15°F.
Parts Commonly Needed
| Part | Notes |
|---|---|
| Flame sensor rod | Measure µA before replacing |
| Integrated control board | Model-specific — cross-reference by part number |
| Limit switch | Match temperature rating exactly |
| Inducer motor | Check capacitor before condemning motor |
| High-pressure switch | Check setting: 410A = 590 psi, R-22 = 380 psi |
| Run capacitor | Check µF with capacitor tester |
Need Replacement Parts?
Before ordering, match the HVAC part to the model number, electrical rating, and failure symptom. Check Carrier rooftop flame sensors, limit switches, capacitors, contactors, and control boards on Amazon - prices and availability change frequently.
Pro tip: Carrier RTU boards store the last 5 fault codes in memory. Access fault history by pressing and holding the LED diagnostic button for 5 seconds on 48/50 Series controls.