Fix P0128 Code: Coolant Thermostat Malfunction – Causes & Repair Guide

Fix P0128 Code: Coolant Thermostat Malfunction – Causes & Repair Guide

P0128 stands for “Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature).” In simpler terms, it means your car’s engine isn’t warming up as quickly as it should.

How Serious Is This Issue?

While P0128 isn’t an immediate threat to your engine, it’s still a serious issue. It can lead to:

  • Increased emissions
  • Reduced fuel efficiency
  • Potential engine damage over time
  • Failed emissions tests

Is It Safe to Drive with This DTC Code?

You can drive short distances with a P0128 code, but it’s not recommended for long trips. The issue could worsen over time, potentially leading to more expensive repairs.

Symptoms and Common Causes of P0128


  • Check Engine Light illumination
  • Longer warm-up times
  • Temperature gauge reading lower than normal
  • Reduced cabin heating
  • Slight decrease in fuel economy

Common Causes:

  • Faulty thermostat (most common)
  • Low coolant level
  • Faulty coolant temperature sensor
  • Wiring issues in the coolant temperature sensor circuit
  • Stuck-open thermostat
  • Faulty ECM (rare)

How Can I Fix It?

  1. Check coolant level and top up if necessary
  2. Inspect and replace the thermostat if faulty
  3. Check and replace the coolant temperature sensor if needed
  4. Inspect wiring for any damage or corrosion

Can I Fix This DTC Code Myself?

If you’re handy with cars, you can tackle some of these repairs yourself:

  • Checking and topping up coolant
  • Replacing the thermostat (on some vehicles)
  • Inspecting wiring

However, if you’re not confident in your skills, it’s best to consult a professional mechanic.

What Parts Might Need Replacement and How Much Will the Repair Cost?

  • Thermostat: $20-$100
  • Coolant temperature sensor: $30-$150
  • Coolant: $20-$50
  • Labor costs: $100-$300 depending on the repair

Will the Repair Be Expensive?

The cost can vary depending on the root cause. Simple fixes like topping up coolant are inexpensive, while replacing a thermostat or sensor can be pricier but still relatively affordable.

Can I Reset the DTC Code Myself?

Yes, you can reset the code using an OBD-II scanner. However, if you haven’t fixed the underlying issue, the code will likely return.

Will It Affect Performance or Fuel Efficiency?

Yes, a P0128 code can negatively impact your vehicle’s performance and fuel efficiency. You may experience:

  • Reduced fuel economy
  • Slightly reduced engine performance
  • Poor cabin heating

How Can I Prevent It in the Future?

  • Follow your vehicle’s maintenance schedule
  • Regularly check coolant levels
  • Address any cooling system issues promptly
  • Use the correct type of coolant for your vehicle

Remember, taking care of this issue promptly can save you money and headaches in the long run. If you’re not comfortable diagnosing or repairing it yourself, don’t hesitate to consult a professional mechanic. Your car (and the environment) will thank you!

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