What Does Carrier ComfortLink Alarm 42 Mean?
On Carrier rooftop units and equipment using the ComfortLink control system, Alarm 42 typically indicates a supply air temperature sensor fault or an invalid temperature input being received by the controller.
When this alarm is active, the unit may:
- Shut down heating or cooling
- Lock out compressors
- Operate erratically
- Fail to maintain space temperature
In most cases, Alarm 42 is caused by a failed sensor or wiring issue, not a major equipment failure.
Common Causes of ComfortLink Alarm 42
The most common causes include:
- Failed supply air temperature sensor (thermistor)
- Loose or damaged sensor wiring
- Shorted sensor leads
- Open circuit in the sensor
- Corroded terminal connections
- Moisture intrusion at the sensor or control board
- Failed input channel on the control board (less common)
Sensor and wiring issues account for the majority of Alarm 42 faults.
How to Troubleshoot Carrier Alarm 42
Follow these steps in order.
1. Power Down the Unit
Turn off power at the disconnect before inspecting sensors or wiring.
2. Locate the Supply Air Temperature Sensor
This sensor is typically mounted in the supply air stream downstream of the evaporator coil.
3. Inspect Wiring and Connections
Check for:
- Loose terminals
- Corrosion
- Broken conductors
- Pinched or rubbed insulation
Correct any visible wiring issues first.
4. Measure Sensor Resistance
Using a digital multimeter, measure the resistance of the sensor and compare it to Carrier’s temperature/resistance chart for that model.
Common findings:
- Infinite resistance → open circuit
- Near-zero resistance → shorted sensor
- Incorrect resistance → failed sensor
5. Check Control Board Input
If the sensor tests correctly, verify reference voltage and input response at the ComfortLink control board.
Only suspect board failure after confirming the sensor and wiring are good.
How to Reset ComfortLink Alarm 42
Once the issue is corrected:
- Restore power to the unit
- Clear the alarm through the ComfortLink interface if available
- If required, cycle unit power to reset the alarm
- Verify normal operation and temperature readings
Some systems will automatically clear the alarm once valid sensor data is restored.
When to Repair vs Replace
Replace the sensor if:
- Resistance readings are incorrect
- The sensor is physically damaged
- Moisture contamination is present
Repair wiring if:
- Connections are loose
- Insulation is damaged
- Corrosion is visible
Control board replacement should only be considered after all other causes are eliminated.
Tools That Help
Technicians commonly use:
- Digital multimeter with resistance measurement
- Manufacturer temperature/resistance charts
- Replacement Carrier-compatible temperature sensor
- Wiring repair kit
FAQ
Can the unit run with Alarm 42 active?
Usually no. Most systems will disable heating or cooling to prevent improper operation.
Is Alarm 42 dangerous?
No, but it can cause loss of cooling or heating if not addressed.
How much does it typically cost to fix?
Sensor replacement is typically low cost compared to major component repairs.
Does Alarm 42 always mean a bad control board?
No. Sensor or wiring faults are far more common than board failure.
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