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Error code reference

Lennox Furnace Error Code E304

LSOM

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What the E304 error code actually means

Your Lennox system is detecting that the compressor, the part responsible for circulating refrigerant for heating or cooling, is not receiving or completing its electrical circuit properly. This could be a failed capacitor, a wiring problem, or a burned-out compressor. A technician will need to test the components to determine whether the compressor itself has failed or if the issue is in the surrounding electrical parts.

Diagnostic procedure

The full E304 procedure

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First diagnostic step
Check capacitor, wiring, hard start kit, replace compressor.
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Most common causes of E304

Ranked by how often they cause this code
  1. 1Failed Run Capacitor
  2. 2Wiring Fault or Loose Connection
  3. 3Missing or Failed Hard Start Kit
  4. 4Open Compressor Winding

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Questions people ask about E304

What does E304 mean on a Lennox furnace?
E304 on a Lennox furnace indicates that the LSOM control has detected an open circuit in the compressor circuit. This means the electrical path through the compressor is not completing correctly. The fault could stem from a failed capacitor, a wiring problem, or a burned compressor winding. A technician should test each component in sequence to isolate the specific failure.
Is it safe to run my furnace with an E304 error code?
The E304 fault specifically affects the compressor circuit, which is part of the cooling or heat pump refrigeration system. The furnace heating function may still operate normally depending on your system configuration, but the compressor will not run while the fault is active. Do not attempt to force the compressor to start repeatedly, as this can cause additional damage to internal components.
Will resetting my Lennox furnace clear the E304 error code?
Resetting the system may temporarily clear the E304 fault code, but if the underlying electrical problem in the compressor circuit has not been corrected, the fault will return when the system attempts to run the compressor again. Repeated reset attempts without addressing the root cause can cause further damage to the compressor windings or other components.
Can I fix a Lennox E304 error myself?
Some portions of the diagnosis, such as visually inspecting wiring for obvious damage, are accessible to experienced DIYers. However, testing and replacing a capacitor involves exposure to potentially dangerous stored electrical charge, and compressor replacement is a refrigerant-involved repair requiring EPA certification. Most homeowners should contact a qualified HVAC technician for this fault.
How much does it cost to repair a Lennox E304 error?
Repair costs vary widely depending on the root cause. Replacing a run capacitor typically falls in the range of $100 to $250 including labor. A hard start kit installation may cost $150 to $350. Compressor replacement is significantly more expensive, generally ranging from $1,200 to $2,800 or more depending on the unit size and labor rates in your area.

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