The P0128 diagnostic trouble code (DTC) is an OBD-II code stored by your vehicle's Engine Control Unit (ECU) when it detects coolant thermostat below regulating temperature. This code is part of the Coolant System system. When your check engine light comes on and a scan tool reads P0128, it means your vehicle needs attention in this area.
OBD-II codes starting with P0 are generic powertrain codes defined by the SAE standard, meaning P0128 has the same basic definition regardless of vehicle make or model. However, the specific causes and repair procedures can vary between manufacturers.
A P0128 code generally allows you to drive short distances to a repair shop, but you should schedule a repair soon. The issue may worsen over time and could lead to reduced performance, poor fuel economy, or secondary damage to other components.