DTC P1258: Engine Over-Temperature Condition: Description
WARNING: This page is about a different car, the 2003 GMC Sierra, 2003 GMC Cab & Chassis Sierra, 2003 Chevrolet Silverado, and 2003 Chevrolet Cab & Chassis Silverado. However, it is still accessible from the selected car via links, so may be relevant.
The ECM uses the ECT sensor to monitor the engine for an over temperature condition. This condition occurs when the coolant temperature is above 270°F (132°C). When an over temperature condition is present, DTC P1258 will set. The ECM will disable two groups of four cylinders by turning OFF the fuel injectors. By switching between the two groups of cylinders, the ECM is able to reduce the temperature of the coolant.