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P0128

Coolant Temp Below Thermostat Regulating Temp

Low
OBD-II emissions
What it means

Engine not reaching operating temperature — typically a stuck-open thermostat.

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Related codes
SPN 110 / FMI 0

Engine Coolant Temperature — Above Normal (Most Severe)

emissionsCritical

General reference only — always confirm against the OEM procedure for your specific engine.