Lennox Air Conditioner Error Code E424

E424 indicates that the liquid line temperature sensor has malfunctioned, registering as either open (infinite resistance) or shorted (near-zero resistance). This sensor is a thermistor mounted on the refrigerant liquid line and provides temperature data to the control board. When its signal falls outside the valid range, the control board triggers E424 and may restrict system operation to prevent damage.

Running the system with an active E424 fault is not recommended. The liquid line temperature sensor provides data used for system protection, and without accurate readings the control board cannot properly detect conditions like refrigerant flooding or overheating. Continued operation could lead to compressor or system damage. The unit should be diagnosed and repaired before resuming normal use.

A power reset may temporarily clear the fault code, but E424 will return if the underlying sensor problem still exists. Because the code is triggered by a measurable electrical condition, an open or shorted thermistor will continue producing the fault on every startup or diagnostic cycle. A permanent fix requires identifying and correcting the sensor, wiring, or connector issue.

Basic diagnostic steps such as inspecting the wiring harness, checking connector condition, and measuring sensor resistance with a multimeter are within reach for knowledgeable DIYers. However, if the sensor pack requires replacement, working on refrigerant-side components may require a licensed HVAC technician depending on local regulations. Confirming the fault with a resistance-to-temperature chart before ordering parts is always recommended.

Repair costs vary depending on the root cause. If only the sensor pack requires replacement, parts and labor typically range from around $100 to $300. If wiring repairs are needed, costs depend on the extent of the damage. A control board replacement, if required, can raise the total cost significantly, often falling in the $300 to $700 range or higher depending on the model.