A check engine light accompanied by engine hesitation and a P0128 code in your 2018 Jeep Cherokee indicates the coolant temperature isn't reaching expected levels within a specified time. This affects fuel delivery calculations and can cause the hesitation you're experiencing as the engine runs in a perpetual warmup mode.
Understanding P0128
P0128 triggers when the coolant temperature sensor reports the engine isn't reaching operating temperature within the ECM's expected timeframe after a cold start. The Cherokee's engine management uses coolant temperature to adjust fuel mixture, timing, and other parameters, so an engine that doesn't warm properly affects driveability.
How Temperature Affects Hesitation
Cold engines receive enriched fuel mixtures. If the ECM thinks the engine is colder than it actually is (or the engine really isn't warming properly), it continues commanding rich mixtures. This can cause hesitation, poor fuel economy, and incomplete combustion symptoms.
Common Causes
A stuck-open thermostat prevents the engine from reaching operating temperature. A faulty coolant temperature sensor sends incorrect readings. Low coolant level affects heat transfer and sensor accuracy. Less commonly, a cooling fan running constantly prevents proper warmup.
Thermostat Primary Suspect
The thermostat regulates coolant flow through the radiator. When stuck open, coolant circulates continuously through the radiator, preventing the engine from warming—especially in cold weather when the radiator efficiently removes heat.