Coolant Thermostat (Coolant Temp Below Thermostat Regulating Temperature)

What is P0128?

P0128 indicates your engine coolant is not reaching the proper operating temperature, typically due to a faulty thermostat stuck open. This is a high-severity powertrain code because it reduces fuel efficiency, increases emissions, and can cause premature engine wear. Based on real owner reports, this issue is particularly common in Jeep and Dodge vehicles, with some models even having fire-related recalls mentioned by owners.

Common Causes

• ●A faulty thermostat stuck in the open position, preventing the engine from warming up properly (most common cause)
• ●Low coolant level due to a leak, as mentioned by a Nissan Altima owner
• ●A defective coolant temperature sensor providing incorrect readings to the engine computer

Symptoms You May Notice

• ●Check Engine Light illuminated on the dashboard
• ●Poor fuel economy due to the engine running too cool
• ●Heater blowing cold air or taking a long time to warm up
• ●Higher than normal emissions during idle, as noted by an owner who had to hold RPMs to prevent a light

How to Fix It

• ●Replace the engine coolant thermostat ($150-$400, moderate difficulty - DIY attempted in only 3% of cases)
• ●Diagnose and repair any coolant leaks, refill, and bleed the cooling system ($200-$600, moderate difficulty)
• ●Replace the engine coolant temperature sensor ($100-$300, moderate difficulty)