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Lennox Furnace Error Code E402

LSOM

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What the E402 error code actually means

Your Lennox system detected that the refrigerant pressure in the outdoor unit was too high, too low, or that the compressor was working too hard, so it shut down to prevent damage. Think of it like a safety switch that trips when something in the cooling or heating refrigerant circuit is out of balance. The system can recover on its own if the problem was temporary, but repeated occurrences mean a technician should inspect the outdoor unit.

Diagnostic procedure

The full E402 procedure

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First diagnostic step
Clears after 4 consecutive normal compressor run cycles.
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Most common causes of E402

Ranked by how often they cause this code
  1. 1Dirty or blocked condenser coil
  2. 2Low refrigerant charge
  3. 3Refrigerant overcharge
  4. 4Condenser fan motor failure
  5. 5Compressor mechanical wear or overload

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Questions people ask about E402

What does E402 mean on a Lennox furnace?
The E402 code indicates a system pressure trip detected by the LSOM module in the outdoor unit. It activates when refrigerant discharge pressure is too high, suction pressure is too low, or the compressor is drawing excessive current. The system shuts down operation to protect the compressor and refrigerant circuit from damage. It will clear automatically after 4 consecutive normal compressor run cycles complete without incident.
Is it safe to run my Lennox system when the E402 code is active?
The E402 fault is a protective shutdown, meaning the system has already stopped compressor operation to prevent damage. Attempting to force the system to run while the fault condition persists risks compressor failure. It is best to leave the system off and contact a technician if the code does not clear on its own within a few normal cycle attempts.
What should a technician check first when diagnosing an E402 on a Lennox system?
A technician should start by inspecting the outdoor condenser coil for blockage and verifying the condenser fan motor is operating correctly. From there, refrigerant system pressures should be measured on both the suction and discharge sides to determine whether the charge level is within specification. Compressor amp draw should also be tested to rule out an overload condition caused by mechanical or electrical issues.
Will resetting the system clear the E402 code on a Lennox furnace?
A manual power reset may temporarily clear the display, but the fault is designed to clear itself only after 4 consecutive normal compressor run cycles. If the underlying pressure or compressor issue is still present, the code will return shortly after the system restarts. Repeated resets without addressing the root cause can accelerate compressor wear.
How much does it cost to repair an E402 fault on a Lennox furnace?
Repair costs vary widely depending on the cause. A condenser coil cleaning may cost between $75 and $200, while a refrigerant leak repair and recharge can range from $200 to $600 or more depending on the refrigerant type and leak location. Compressor replacement, if required, is typically the most expensive repair and can range from $800 to $2,500 or higher including labor.

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