Fault Tree

OBD-II differential diagnosis

P0128 — Coolant thermostat below regulating temperature

The engine is not reaching operating temperature in the expected time. It still runs, but it runs rich, uses more fuel, and wears faster.
System
Powertrain
Definition
Generic (SAE-defined, all manufacturers)
Subsystem
Fuel and air metering
Severity
minor

Causes, by prior probability

CausePriorRepair
Thermostat stuck open
Coolant temperature plateaus well below 85°C on a highway run.
 62% $212
Coolant temperature sensor out of range
Compare sensor reading to an infrared thermometer on the housing.
 24% $103
Low coolant level
Check level cold. Also explains intermittent heater output.
 14% $61
These are priors from the code alone. A scan's freeze frame reorders them, which is the entire point of capturing it.