HVACopediaCarrier · Air conditioner troubleshooting
Two-stage unit not changing stages - Carrier 24ANB/25HNB/25HCB6
What is going on
Unit runs but capacity does not seem to change between low and high stage.
Likely causes
- Unloader solenoid not getting 24 V DC
- Solenoid plug without internal rectifier
- Open or grounded unloader coil
- Control or thermostat not calling Y2
Check in this order
- Watch suction pressure it should fall roughly 3-10 percent going to high stage; do not judge by liquid pressure.
- Compressor amps should rise 20-45 percent in high stage.
- Cycle low/high at 30 s or longer intervals and read amps 5 s after the change; at least 20 percent change is expected.
- No change unplug the solenoid plug and check for 24 V DC while high stage is called.
- Voltage good measure the coil; about 330 or 1640 ohms is normal - open or grounded means compressor replacement.
- Only use a solenoid plug with the built-in rectifier.
Applies to: 24ANB1, 24ANB7, 25HNB6, 25HNB9, 25HCB6, CH17
More air conditioner guides
- Amber STATUS or green COMM light not on
- Compressor will not run - winding and electrical checks - Carrier split systems
- Delay before the compressor restarts
- Hissing from the outdoor unit after it shuts off
- Outdoor unit unresponsive after several faults
- Superheat too high or too low on TXV system - Carrier split systems
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Source: Carrier Residential AC & HP Application Guideline and Service Manual 24-25-9SM (text export), Unloader Test Procedure. Written in our own words; check the manual for your exact model.
For licensed technicians. Gas, electrical and refrigerant work must be done by a qualified, licensed person. Shut off power and gas before servicing, never bypass a safety control, and follow the manufacturer’s manual for the exact model; it and local codes always take priority. HVACopedia is not affiliated with any manufacturer; brand and model names only identify the equipment.