High pressure lockout for 4 hours, elevated from fault 31 when the stage (speed) can no longer be lowered.
Homeowner part this code names: furnace air filter.
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Carrier 84 is a high-pressure lockout for 4 hours, elevated from fault 31 when the stage can no longer be lowered. The LED flashes 8 short and 4 long. Causes listed for 84, beyond the shorter list code 31 points at, include low indoor airflow in heating, an overcharged system, a reversing valve stuck in cooling during a heating call, a service valve left closed, a loose or disconnected high-pressure-switch harness, a restricted filter drier or expansion device, non-condensables, a faulty pressure switch, and a sudden supply-voltage change. In heating, the homeowner check is the indoor coil and the filter. Also check the outdoor coil for ice or debris. Charge, valves, the harness, the filter drier, and the expansion device are technician work. The 4-hour lockout is the control stopping the retries that code 31 was still allowing.
Likely causes
- Low indoor airflow in heating mode
- Overcharged system
- Reversing valve stuck in cooling (heating mode)
- Service valve left closed
- Loose or disconnected high pressure switch harness
- Restricted filter drier or expansion device
- Non-condensables in the system
- Faulty pressure switch
- Sudden supply voltage change
Safe checks for 84
- In heating mode, check the indoor coil and filter for restriction by debris and clean if necessary (the manual also says to troubleshoot the indoor fan motor)
- Check the outdoor coil for clogging by ice or debris and clean or de-ice it if necessary (the manual also says to troubleshoot the outdoor fan motor)
- Technicianmake sure the service valves are open, check the pressure switch harness and connection, check the filter drier and expansion device, and check charge
Capacitors can hold a charge after the power is off. Safety terms.
When to call a pro
Call a licensed HVAC technician: lockout 84 involves refrigerant-side checks.
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Parts & tools for this fix
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Furnace and AC air filter: guide
In this code: In heating mode, check the indoor coil and filter for restriction by debris and clean if necessary (the manual also says to troubleshoot the indoor fan motor)
AC and heat pump coil cleaning supplies: guide
In this code: In heating mode, check the indoor coil and filter for restriction by debris and clean if necessary (the manual also says to troubleshoot the indoor fan motor)
See the guide for what to look for when buying one.
Multimeter for appliance and HVAC diagnosis: guide
In this code: Sudden supply voltage change
This one is for a technician to test or replace, so we only explain it and do not link to buy it.
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