Rheem HVAC Error Code 20: How to Fix It

Q: What does Rheem error code 20 mean?

Rheem error code 20 means the outdoor air temperature sensor circuit is open. The control board expects a continuous resistance signal from the sensor, and an 'open' reading means it's receiving no signal at all — usually because the sensor is disconnected, the wire is broken, or the sensor has failed.

Q: Can I still use my HVAC system with error code 20 active?

Your system may continue to operate in a limited capacity, but it could lock out certain modes like heat pump operation or restrict efficiency-based controls that rely on outdoor temperature data. Running the system long-term with a faulty sensor can stress components and lead to additional faults, so it's best to address the issue promptly.

Q: How do I test an HVAC outdoor air sensor with a multimeter?

Disconnect the sensor plug from the control board and set your multimeter to resistance (Ohms). Touch the probes to the two sensor terminals. At room temperature (around 77°F/25°C), a good sensor reads approximately 10,000 ohms. A reading of OL (open loop/infinite) means the sensor is failed open — the same fault that triggers error code 20.

Q: How much does it cost to replace a Rheem outdoor air temperature sensor?

The sensor part itself typically costs between $15 and $50 depending on your specific Rheem model. If you hire a technician to do the replacement, expect to pay an additional $75 to $150 in labor, making the total repair roughly $90 to $200 in most areas.

Q: Why does my Rheem HVAC keep showing error code 20 after I replaced the sensor?

If the error persists after replacing the sensor and confirming solid wiring connections, the problem may be with the control board itself — specifically the sensor input circuit on the board. It's also worth double-checking that the replacement sensor is the correct type and resistance value for your Rheem model. At this point, a licensed HVAC technician should inspect the control board.

Abstract

Rheem HVAC error code 20 indicates that the outdoor air temperature sensor circuit is open, meaning the control board is not receiving a valid signal from the sensor located in the outdoor unit. This typically happens when the sensor wire is disconnected, broken, or the sensor itself has failed. Left unresolved, the system may run inefficiently, lock out certain operating modes, or shut down entirely to protect equipment.