30/07/2026 6:12 am
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Automotive Q&A
P0128 with coolant temperature leveling at 74°C on the highway
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A 2014 Jeep Cherokee 3.2 sets P0128 in cool weather. After an overnight soak, coolant and intake-air temperatures agree within 2°C. During a 20-minute highway drive, coolant rises normally but levels near 74°C; the upper radiator hose begins warming early and cabin heat is only moderate. The cooling fans are not commanded.
Does this pattern support a thermostat that is opening early, or should the temperature sensor still be tested?
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31/07/2026 8:23 am
The matching cold-soak values make a large sensor offset unlikely. Compare scan temperature with a contact measurement at the outlet as the engine warms and watch the rate of hose-temperature change. A thermostat leaking or opening early creates a gradual radiator warm-up before the specified opening range and prevents the engine from reaching target under road airflow.
Confirm coolant level and circulation first. If measured and reported temperature agree and the radiator receives heat too early, thermostat control is supported. Recheck warm-up on the same route after repair rather than clearing the code and idling in the bay.
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01/08/2026 8:51 am
Scan temperature and the contact probe stayed within 3°C. The radiator inlet warmed from the beginning instead of at the opening point. The replacement thermostat allowed coolant to reach 91°C on the same route, cabin heat improved and the P0128 monitor passed.