Lennox Heat Pump Error Code Green FAST Flash / Red ON

This code indicates a discharge line temperature lockout caused by five consecutive temperature faults during a single Y1 demand cycle. The control board detected either a shorted discharge line sensor or actual high discharge temperatures five times before triggering the lockout. The lockout is a protective measure to prevent compressor damage from sustained high-temperature operation.

Repeatedly resetting the unit without fixing the underlying problem puts the compressor at risk. The lockout system exists specifically to prevent compressor failure from overheating, and bypassing it through repeated resets can shorten compressor life significantly. If the fault recurs after the first reset, the unit should remain off until a technician completes a proper diagnosis.

Repair costs vary widely depending on the root cause. Replacing a faulty discharge line temperature sensor typically runs in the range of $150 to $350 including labor. Refrigerant recharge or leak repair can range from $200 to $600 or more depending on the refrigerant type and amount needed. Compressor replacement is the most expensive scenario, often ranging from $1,200 to $2,500 or higher depending on the unit size and local labor rates.

Resetting by cycling 24V power or shorting the TEST pins for 1 to 2 seconds will clear the strike count and allow the unit to operate again, but the lockout will return if the underlying problem is not corrected. Each new Y1 demand cycle starts a fresh strike count, so a marginal system may take a few cycles before locking out again. The root cause, whether a faulty sensor or a refrigerant system issue, must be diagnosed and repaired for the lockout to stop recurring.

Start by checking the resistance of the discharge line temperature sensor and comparing it to the rated specification for your model. A shorted sensor will read near zero ohms or significantly below the expected value at a known temperature. If the sensor checks out within spec, connect refrigerant manifold gauges and evaluate system pressures during operation to identify low charge, restriction, or compressor issues.