Lennox HVAC Error Code 133

Error code 133 on a Lennox system means the iComfort thermostat has detected a problem with the air-handler outdoor sensor. The thermostat monitors the sensor's resistance output continuously, and when that reading falls outside the expected range for the current outdoor temperature, it triggers this fault. The system flags the error to alert you that outdoor temperature data may be unreliable, which can affect certain operating functions.

Error code 133 does not indicate a safety hazard such as a gas leak or electrical fault. However, running the system with a faulty outdoor sensor means the iComfort controls may not optimize performance correctly, since they rely on accurate outdoor temperature data for certain functions. It is best to have the sensor diagnosed and repaired promptly to ensure the system operates as intended.

Resetting the thermostat may temporarily clear the fault display, but the error will return if the underlying sensor problem has not been resolved. Code 133 is generated because the thermostat continuously monitors the outdoor sensor's resistance, so any persistent sensor fault will retrigger the code after a reset. The sensor and wiring need to be tested and repaired or replaced to permanently clear the error.

A knowledgeable DIYer can inspect the sensor wiring for visible damage, check connector terminals for corrosion, and use a multimeter to measure the sensor's resistance. Comparing that resistance value to the temperature-resistance chart in the unit's installation documentation will confirm whether the sensor has failed. However, if the sensor pack needs replacement or the control board is involved, most homeowners will want to have a qualified HVAC technician complete the repair.

If the repair involves only reseating a connector or cleaning corroded terminals, costs are primarily the technician's diagnostic and labor fee, typically in the range of $75 to $150. If the outdoor sensor pack requires replacement, parts and labor combined generally run between $100 and $300 depending on your region and the specific unit model. Control board replacement, if needed, would be more expensive and could range from $300 to $700 or higher.