Trouble codes › P0128

P0128 Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature)

Cooling

P0128 means the engine isn't reaching normal operating temperature, almost always a thermostat stuck open.

After the engine has run long enough to warm up, the engine computer expects the coolant to reach the thermostat's opening temperature. If it's taking too long or never gets there, it sets P0128. Most of the time the thermostat is stuck open.

Can I keep driving? Yes, for now.

Safe to drive; it won't overheat from this. Expect weak cabin heat in winter and somewhat worse fuel economy, and fix it before cold weather.

Common causes

  1. A thermostat stuck open or opening too early (by far the most common)
  2. A faulty engine coolant temperature sensor reading low
  3. Low coolant level
  4. A radiator fan running all the time

Symptoms

  • The temperature gauge stays low or takes a long time to come up
  • Weak heat from the heater, especially at highway speed in cold weather
  • Slightly worse fuel economy

Common on: Ram 1500, Jeep Grand Cherokee, Jeep Wrangler, Hyundai Elantra, Dodge Durango

Known problems: Ford 1.5L and 1.6L EcoBoost coolant loss, overheating and misfires

What to check first

  1. Check the coolant level with the engine cold.
  2. Watch the temperature gauge (or coolant temperature on a scan tool) on a 15-20 minute drive: if it climbs slowly and drops on the highway, the thermostat is likely stuck open.
  3. Check that the radiator fan isn't running from a cold start.
  4. Replace the thermostat with the correct temperature rating for your engine, and bleed air from the cooling system afterward as your service information describes.

What does P0128 mean on your vehicle?

The likely cause depends on your engine. iRepair researches your exact year, make, model and engine (service bulletins, recalls and owner reports) and tells you what to check first. Your first 3 questions are free.

Ask iRepair about P0128

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Related codes

General guidance for P0128 on most vehicles. Some makers use the same code a little differently; confirm in the service information for your vehicle, put safety first, and see a professional when in doubt.