Slow warm-up or a thermostat stuck open: DTC P0128 on the Chrysler 300C and Dodge

Your Chrysler 300C has been running for a good while, but the temperature needle stays low, lower than usual. When you turn the heater on, the air is not as warm as it used to be. Then the check engine light comes on, and the scan result shows P0128.

Many owners shrug this code off because the engine is not running hot. It is true that P0128 does not mean you have to stop right away, but an engine that stays cold too long uses more fuel, and the cause can be more than just the thermostat. On the Chrysler 300 and its Dodge siblings, there is also a history of manufacturer service bulletins that specifically discuss this code. This article explains what P0128 means, what is specific to the 300 and its Dodge siblings, and a safe order of checks.

P0128 is a generic OBD-II code with the title "Coolant Thermostat Below Regulating Temperature", or coolant temperature below the thermostat operating temperature. The engine computer monitors the coolant temperature sensor from the moment the engine is started. If within a certain time that temperature does not reach the expected range, the computer concludes something is making the engine stay cold too long and stores P0128.

The thermostat holds back the flow of coolant to the radiator until the engine reaches operating temperature. If the thermostat is stuck in the open position, coolant keeps flowing to the radiator from the start, so the engine warms up slowly. One source says the engine temperature should be close to 82–105 °C after about ten minutes of driving. This code does not prove the thermostat is broken: low coolant, a biased temperature sensor, or a radiator fan that keeps running can also cause it.

The first generation Chrysler 300 was built on the LX platform, which was also used by the Dodge Charger, Dodge Challenger and Dodge Magnum. Its engine choices include the 2.7 L V6, 3.5 L V6, 5.7 L Hemi V8, 6.1 L Hemi V8 and, in some markets, the 3.0 L diesel. The second generation (LD platform, 2011 to 2023 models) uses among others the 3.6 L Pentastar V6, 5.7 L Hemi V8 and 6.4 L Hemi V8.

Because the most relevant cause of P0128 differs by engine, the first step is to confirm which engine is in your car. The name "300C" is also used differently by market and year, and the versions sold in one country are not always the ones sold in another. Check the engine code from the vehicle documents or the label in the engine bay before following advice that is specific to one engine.

According to a summary of a Chrysler service bulletin, there is a bulletin about P0128 on the 3.5L engine that applies to the 2009 to 2010 Dodge Challenger and the 2008 to 2010 Chrysler 300, Dodge Charger and Dodge Magnum. The summary says the part of the engine block that is supposed to hold the thermostat may not hold it well. The thermostat then tips into the opening in the block, so coolant flows past it without going through the thermostat. With the thermostat in the wrong position, the system stores P0128.

According to that summary, the repair is to install a shim on the thermostat on cars that have not yet turned on the warning light, and to replace the thermostat housing with a new kit (thermostat, housing and shim) for cars that have already stored this code. The summary also says the technician is asked to check and repair other codes first if there are any. If your Chrysler 300 or Dodge has the 3.5L engine and falls in that year range, it is worth telling the workshop about this bulletin before the thermostat is simply replaced. Another source also notes several Chrysler bulletins for a number of Jeep and Dodge models that set reprogramming of the engine computer as the repair for P0128, so the workshop needs to match the VIN against the bulletins that apply.

On the Chrysler 300 with the 3.6 L Pentastar engine, offered since the 2011 model year, the most often named cause of P0128 is a thermostat stuck open. Other causes that need to be ruled out are low coolant, a faulty radiator fan or water pump, an inaccurate coolant temperature sensor, or a faulty engine control module.

The thermostat and its housing are at the front of the engine. After replacement, the cooling system must be refilled and bled of air correctly, because trapped air can damage the engine when hot. This work should only be done after the engine is completely cold.

• A thermostat that is stuck open, or that sticks now and then.
• A thermostat seal that does not seal, so coolant passes a thermostat that is actually closed.
• Low coolant.
• A coolant temperature sensor (ECT) that reads too low, or a corroded connector.
• A radiator fan that will not turn off.
• Engine module software that needs updating, if there is a bulletin for that car.

1. Confirm the engine type. The 3.5L engine in the years covered by the bulletin has its own repair path, while the 3.6 Pentastar usually points to the thermostat and its housing.
2. Read all codes. According to the summary of the Chrysler bulletin, other codes are repaired first before handling P0128.
3. Check the coolant level when the engine is cold. Never open the cooling system when the engine is hot.
4. Monitor the temperature on a scan tool. Record the temperature when the engine is cold and the time needed to reach operating temperature. A temperature that rises and then drops at cruising speed points further toward a thermostat stuck open.
5. Test the temperature sensor. When the engine is cold, its reading should be close to the outside air temperature.
6. Replace the thermostat with the manufacturer procedure, then fill and bleed the cooling system correctly.

If the engine runs well, the coolant level is sufficient, and the temperature needle is only slow to rise, P0128 is generally not a reason to stop right away. Watch the temperature needle during the trip and schedule an inspection, because an engine that stays cold too long uses more fuel.

The situation is different if the temperature needle goes high instead, the coolant level drops, or there is steam from the engine bay. In that condition, pull over in a safe place, turn off the engine, and do not open the radiator cap or the reservoir while hot, because the system is pressurized.