System Diagnosis
Low pressure control open.
Rheem AC and Heat Pump
Error Code 21
Troubleshooting guide for the Rheem AC and Heat Pump system fault 21.
Disclaimer: This information is for general reference. Error codes can vary by model and production year. Always refer to your machine's specific manual and the information printed on the unit itself.
Potential Causes & Solutions
- Low refrigerant charge, refrigerant leak, frozen indoor coil, dirty filter, restricted airflow, blocked ducts, expansion device issue, or faulty low-pressure control.
Verify Before Replacing Parts
- Confirm the exact Rheem model family and ac and heat pump context before acting on a generic code description.
- Verify airflow first, then confirm operating pressures and temperature relationship before adjusting charge.
- Look for restrictions, iced coils, or dirty heat-transfer surfaces that can mimic a charge problem.
- After the correction, run the system long enough to confirm the code does not return under normal load.
Field Signals to Confirm
What usually confirms it
Pressure and temperature readings that stay out of range after airflow is verified.
What can fool you
Dirty coils or airflow restrictions can look like a charge problem until the air side is cleaned up.
When to Escalate the Call
- You suspect a restriction, leak, or compressor issue that needs deeper system evaluation.
- The readings do not normalize after airflow and coil conditions are corrected.
Not sure which system this Rheem code is on?
ACLogics has guided diagnostics for 57 Rheem fault codes and asks the right questions to narrow it down.
