P0128 on GMC

The 2007-2016 Acadia uses an electric power steering rack that is prone to intermittent and permanent failure. The steering assist can fail suddenly with no warning, requiring significantly more physical effort to steer — a dangerous condition at highway speeds. NHTSA received over 800 complaints and opened an investigation. The failure mode involves the EPAS motor and control module overheating, producing a Service Power Steering warning. Failures are more common in warm climates and on vehicles that have been through multiple hot/cold cycles.

The 4.3L Vortec V6 uses a lower intake manifold gasket that develops coolant and oil leaks where it seals the intake to the block. The OEM gaskets use a plastic carrier that warps over time, allowing coolant to mix with oil or leak externally.

On the Sierra 1500, P0128 means the coolant did not reach the expected regulating temperature within the time/airflow the ECM predicted — almost always a thermostat stuck open (or partly open) so the engine runs too cool. The GM thermostat on the 5.3L/6.2L is integrated into a plastic housing on the engine, and the common failure is the thermostat losing its ability to close, letting coolant circulate continuously. Drivers notice the temp gauge sitting low, weak or slow-to-warm cabin heat (especially in winter), and worse fuel economy because the ECM stays in open-loop enrichment longer. A stuck-open thermostat is by far the most frequent cause; a failed/biased coolant temperature sensor is the secondary suspect and should be verified with live data before parts are thrown at it.

The factory water pump on 6.6L Duramax engines (LBZ, LMM, LML, L5P) is known to develop weep hole leaks and bearing failures, resulting in coolant loss and potential overheating. The plastic impeller on earlier units can also separate from the shaft, causing sudden loss of coolant circulation. GM issued TSB 07-06-02-002 addressing water pump replacement procedures for LBZ/LMM applications.

The engine-driven water pump develops bearing/impeller wear or seal leaks, allowing coolant to weep (often visible as dried Dex-Cool on the timing-chain cover or flung from the pump pulley) and eventually causing overheating. On both the 2.4L Ecotec and 3.6L V6, the pump is timing-chain-cover-mounted, making replacement labor-intensive and comparatively expensive. Neglecting a leaking pump can damage timing components.

A worn or leaking factory thermostat that stays open (plus a failing engine coolant temperature sensor) prevents the engine from reaching normal operating temperature. The ECM interprets the low/erratic reading as a sensor fault and enters a protective fail-safe mode: the electric cooling fans run at full speed continuously, the temperature gauge may drop to zero or read erratically, the A/C compressor may be disabled, and fuel economy suffers. It is a common and well-documented complaint across 2015-2019 5.3L/6.2L trucks and SUVs.