Lennox Furnace Error Code E303

E303 is a fault code generated by the Load Shed Output Module (LSOM) indicating either a locked compressor rotor or compressor short cycling, meaning the compressor is running for less than 4 minutes per cycle. A locked rotor means the compressor motor is receiving power but cannot rotate. Both conditions signal a potentially serious problem with the compressor or supporting components that requires professional diagnosis.

Resetting the furnace may temporarily clear the E303 code, but if the underlying fault with the compressor or supporting components is not repaired, the code will return quickly. Repeatedly resetting and restarting a compressor in a locked rotor condition can cause serious and permanent damage to the motor windings. It is best to leave the system off and have a technician diagnose the root cause before attempting a reset.

Repair costs vary widely depending on the root cause. Replacing a failed capacitor is typically a relatively low-cost repair, often in the range of $150 to $400 including labor. A contactor replacement falls in a similar range. If the compressor itself is seized or failed, replacement costs can range from $1,200 to $2,800 or more depending on the unit size and refrigerant type. A refrigerant recharge and leak repair generally falls somewhere in between those two extremes.

Most of the underlying causes for E303 require professional tools and certifications to address properly. Refrigerant handling requires an EPA Section 608 certification, and safely measuring locked rotor amperage requires a clamp meter and knowledge of electrical safety. A homeowner can visually inspect the contactor for obvious burn marks, but capacitor replacement and refrigerant work should be left to a certified HVAC technician.

No, continuing to operate the system with an active E303 code is not recommended. A locked rotor condition creates extremely high current draw that can overheat and burn out compressor windings in a very short time. Even if the fault is short cycling, repeated starts and stops stress the compressor motor and can cause it to fail prematurely. Shut the system down and schedule a service call promptly.