P0128 Jeep Grand Cherokee: Coolant Temperature Below Thermostat Regulating Temperature
Quick Answer: What is P0128?
On a Jeep Grand Cherokee, the P0128 code means the engine is taking too long to warm up or isn't reaching its target operating temperature (usually around 190°F-210°F). The computer (PCM) monitors how fast the temperature rises; if it stays too cool for too long, it assumes the thermostat is stuck open.
Common Symptoms
- Temperature Gauge Stays Low: The needle barely moves or takes 20+ minutes to reach the middle.
- Decreased Fuel Economy: The engine stays in "Open Loop" mode, burning more gas.
- Weak Heater: The air from the vents feels lukewarm instead of hot.
- Auto Start/Stop Disabled: Newer Jeeps will disable this feature if the engine isn't at the correct temp.
Most Likely Causes
- Stuck Open Thermostat (85%): The internal spring or seal has failed, allowing coolant to flow constantly.
- Faulty Coolant Temp Sensor (ECT): Sending "lazy" or incorrect data to the PCM.
- Low Coolant Level: Air pockets preventing the sensor from getting an accurate reading.
- Cooling Fan Malfunction: The fan is running at high speed constantly, over-cooling the engine.
Detailed Repair Cost Breakdown
Estimates for Jeep Grand Cherokee (3.6L V6 and 5.7L V8) in the US market:
| Component / Task | Aftermarket Part | OEM Mopar Part | Labor Time | Difficulty |
|---|---|---|---|---|
| Thermostat Housing Assy | $40 - $65 | $95 - $140 | 1.0 - 1.5 hrs | Easy (DIY) |
| Coolant Temp Sensor | $20 - $35 | $55 - $85 | 0.5 - 1.0 hr | Easy |
| Coolant Flush/Top-off | $25 (Fluid) | $45 (OAT Fluid) | 0.5 hr | Essential |
How to Fix P0128 on a Jeep Grand Cherokee
1. Check Coolant Level and Type
Ensure the overflow tank is full. Newer Jeeps use OAT (Purple) coolant; never mix it with the older HOAT (Orange) or universal green coolant, as it will gunk up your heater core and radiator.
2. The "Hose Feel" Test
Start your Jeep from a cold state. Feel the upper radiator hose. It should stay cold for several minutes while the engine warms up. If it starts getting warm immediately after starting the engine, your thermostat is stuck open and must be replaced.
3. Replacing the Thermostat
On the 3.6L Pentastar engine, the thermostat is located right at the front-top of the engine. It is a very easy DIY job (two 10mm bolts). Pro Tip: Buy the entire housing assembly, not just the thermostat, as the plastic housings on Jeeps are known to crack and leak over time.