Lennox Furnace Error Code E300

E300 on a Lennox furnace is a LSOM fault indicating the air conditioner compressor ran for more than 18 hours continuously in cooling mode. The compressor's internal safety protector has opened to prevent damage from overheating or excess current draw. This code requires a technician to inspect head pressure, supply voltage, the contactor, disconnect, circuit breakers, fuses, and all associated wiring before restarting the system.

No, you should not attempt to force the system to run while E300 is active. The compressor protector opened specifically to prevent compressor burnout or mechanical failure, and bypassing this condition risks destroying the compressor entirely. Leave the system off until a qualified technician has identified and corrected the underlying fault.

The most frequent causes of E300 are high refrigerant head pressure from a dirty condenser coil or failed condenser fan, and low supply voltage that forces the compressor to draw excessive current. Faulty contactors, open disconnects, tripped breakers or blown fuses, and damaged wiring to the outdoor unit are also common contributors. A technician will check all of these components as part of diagnosing this fault.

The compressor protector that triggers E300 will often reset on its own after the compressor cools down, which may temporarily clear the fault. However, simply resetting the system without identifying and correcting the root cause will almost certainly result in the fault returning, and repeated trips can cause permanent compressor damage. A proper diagnosis of the outdoor unit, refrigerant system, and electrical supply is necessary before relying on a reset.

Repair costs for E300 vary widely depending on the root cause. A simple fix such as cleaning the condenser coil or replacing a contactor may run in the range of $100 to $400. If the compressor itself has sustained damage and requires replacement, costs can range from $1,200 to $2,800 or more depending on the unit size and labor rates in your area. Getting a diagnostic inspection first will clarify what repairs are actually needed.