P0128 Diagnostic and Fix for 2006 Chevrolet Trailblazer with Engine Running Too Cool
A few years ago, my 2006 Chevrolet Trailblazer experienced a headlight relay failure after driving through a large puddle. Replacing the relay resolved that issue, and the headlights have been functioning properly since. Recently, I've started receiving persistent diagnostic trouble codes P0128 and P0128 PD (Engine Running Too Cool). Initially, I suspected the radiator fan, as it runs continuously—even when the engine is cold and the vehicle is first turned on. I replaced the fan relay, assuming it might have been damaged similarly to the headlight relay, but the codes persist. My next step is to drain the coolant and replace the thermostat, as a stuck-open thermostat could cause the engine to run too cool. I've also read that the Engine Coolant Temperature (ECT) sensor may be faulty. Is there any way to test this sensor without professional tools or at home? Would replacing just the thermostat resolve the issue? Any insights or troubleshooting tips for this specific 2006 Chevy Trailblazer with P0128 would be greatly appreciated!
A thought: since you're draining coolant to replace the thermostat, consider also replacing the ECT sensor. There are no other diagnostic codes reported? The radiator fan is running continuously even when the engine is cold and the vehicle is first started. *Note: For the 4.3L engine in this Chevrolet model, the ECT sensor is relatively inexpensive—testing it at home or swapping it out may be a cost-effective solution.