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Randall C
Randall C, ASE Master Tech
Category: Chevy
Satisfied Customers: 12720
Experience:  ASE Master, L1, 32 years, Diagnostic / Electrical Specialist
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code P0128 coolant temp below thermostat regulating temp, what

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code P0128 coolant temp below thermostat regulating temp, what does this mean and what is the fix? 2004 chev. trailblazer w/4.2 engine

Hello, below is the description of 128 code. basically, the ECM is not seeing the engine get up to temp quick enough./ A scan tool and lengthy diag chart are used to diagnose but most parts of the chart lean to bad ECT or engine coolant temp sensor


DTC P0128

An engine coolant temperature (ECT) sensor monitors the temperature of the coolant. This input is used by the powertrain control module (PCM) for engine control and as an enabling criteria for some diagnostics.

The air flow coming into the engine is accumulated and used to determine if the vehicle has been driven within the conditions that would allow the engine coolant to heat up normally to the thermostat regulating temperature. If the coolant temperature does not increase normally or does not reach the regulating temperature of the thermostat, diagnostics that use ECT as enabling criteria may not run when expected.

This DTC will only run once per ignition cycle within the enabling condition. If the PCM detects the calibrated amount of air flow and engine run time have been met, and the ECT has not met the minimum thermostat regulating temperature, DTC P0128 sets.

Randall C and 5 other Chevy Specialists are ready to help you
Hello, I was contacted by one of the other experts that shed some light. He has 99% of the time found the thermostat bad with a code 128. I wanted to add that information for you. thanks