When your 2020 Lexus RX overheats with the AC running but stays cool with it off, the air conditioning system is adding load the cooling system can't handle. This doesn't necessarily mean the AC compressor is faulty - the cooling system may be marginal and only fails under the additional thermal load.
How AC Load Causes Overheating
The AC compressor adds 5-10 horsepower load to the engine and the condenser adds heat in front of the radiator. On marginally performing cooling systems, this extra burden tips the balance toward overheating. The RX's 3.5L V6 generates significant heat, and when cooling capacity is already reduced, AC operation becomes the trigger.
Common Contributing Factors
A partially clogged radiator may cool adequately at idle but fail under AC load. The condenser fan may not be running at high speed when needed. The auxiliary electric cooling fan should activate when AC is on but may have failed. Reduced coolant flow from a weak water pump becomes evident under high load.
Diagnostic Approach
Monitor temperatures with AC cycling on and off using an OBD scanner. Watch how quickly temperature rises when AC engages. Check that both cooling fans run at high speed with AC on. A thermal camera can reveal hot spots in the radiator indicating blockages.
Why This Affects Summer Driving
Marginal cooling systems may work fine in mild weather but fail when ambient temperatures climb and AC runs continuously. The combination of high ambient heat, AC compressor load, and condenser heat creates the perfect overheating conditions.