P0128 on a Jeep: Causes, Fixes & Repair Cost

🟠 Moderate severity · drivable

Safe to drive if the coolant level is fine, but the engine running cool hurts fuel economy and heater output, so plan the repair soon.

What P0128 means on a Jeep

On a Jeep, P0128 means the engine computer has detected that the engine isn't warming up to its normal operating temperature in the expected time. On the popular 3.6L V6 found in many Jeeps, the thermostat housing is a known weak point and a common cause of this code.

Is P0128 common on Jeeps?

P0128 shows up on Jeeps like the Wrangler, Grand Cherokee, Cherokee, and Compass. The 3.6L engine's thermostat housing is a well-known failure point, so many Jeep owners replace the housing and thermostat together.

Commonly reported on: Wrangler, Grand Cherokee, Cherokee, Compass.

Symptoms

Check engine light is on
Temperature gauge reads below normal
Heater blows lukewarm air
Coolant level drops slowly

Common causes

most common: Thermostat stuck open
common: Cracked thermostat housing
common: Low coolant from a housing or hose leak
less common: Faulty coolant temperature sensor

How to fix it

Fix	DIY?	Typical cost
Top up the coolant and inspect the housing and hoses	Yes	$10–30
Replace the thermostat and housing	Yes	$80–200
Replace a leaking coolant hose	Yes	$30–90