American Standard Air Conditioner Error Code 8

Error code 8 on an American Standard PTHP unit indicates an outdoor air thermistor fault. This means the control board is receiving a signal from the outdoor temperature sensor that falls outside the expected resistance range. The code only appears on PTHP models because those units actively use outdoor air temperature data to manage heat pump and defrost operation.

A power reset may temporarily clear code 8, but if the underlying thermistor or wiring fault still exists, the code will return once the control board re-evaluates the sensor signal. A reset is useful for confirming whether the fault is intermittent, but it does not substitute for diagnosing and correcting the root cause.

If only the outdoor air thermistor needs replacement, parts costs are generally low, typically in the range of $15 to $50 for the sensor, plus labor if a technician is involved. If the control board requires replacement, costs rise significantly, often ranging from $150 to $400 or more depending on the model and labor rates in your area. Diagnosing the wiring harness first can help avoid unnecessary part replacement.

Some causes of code 8 are accessible to a knowledgeable DIYer, such as inspecting and cleaning corroded wiring connectors or reseating a displaced thermistor. Replacing the thermistor itself is also a relatively straightforward repair if you can source the correct part. However, if the control board input circuit is at fault, board diagnosis and replacement is better handled by a qualified technician.

Operating the unit with an active thermistor fault is not recommended, particularly in heating mode. Without reliable outdoor air temperature data, the heat pump may run inefficiently, fail to initiate defrost cycles correctly, or operate outside its intended parameters. Continued use in this condition could cause additional wear on the refrigerant system.