What code does a bad thermostat throw?

Author: Jacey Mertz  |  Last update: Wednesday, October 29, 2025

P0128: Coolant Thermostat Temperature Below Regulating Temperature. When your check engine light is illuminated due to trouble code P0128, you may have an issue with your engine thermostat.

Will a bad thermostat show a code?

Yes, a faulty thermostat can trigger the check engine light in your vehicle. The thermostat regulates the engine's temperature by controlling the flow of coolant. If it fails to open or close properly, it can lead to overheating or underheating of the engine.

What is obd code for bad thermostat?

P0128 is an extension of OBD-II code P0 which indicates a problem with your car's coolant thermostat.

Can I still drive my car with a P0128 code?

Although it may be possible to drive short distances with code P0128 set, you should get the issue fixed by a professional right away. Doing so will help ensure your vehicle keeps running at its best while also helping to prevent any additional damage.

What is the code for a bad temperature sensor?

The P0118 code refers to issues with the Engine Coolant Temperature (ECT) circuit and its accompanying sensor. The ECT is a thermistor, or an electrical resistor, whose resistance is significantly reduced by heating.

EricTheCarGuy Explains Check Engine Codes - P0128 Coolant Below Thermostat Regulating Temperature

Can you drive with a bad thermostat?

If the old thermostat is positioned at the bottom of the radiator, you have no choice because all the coolant will drain by itself. If you drive with a bad thermostat, your car could overheat, causing severe and expensive engine damage.

How do you diagnose a bad temperature sensor?

Symptoms of bad temperature sensor
  1. Erratic Readings. Sudden jumps or drops in temperature readings. ...
  2. Open Circuit: Display shows a very high or infinite temperature. ...
  3. Short Circuit: Display shows a very low or negative temperature. ...
  4. Drift: ...
  5. Noise or Interference: ...
  6. Physical Damage:

What happens if you drive with a bad coolant temperature sensor?

If the coolant temperature sensor malfunctions, it could cause your car engine to get overheated. While sometimes a faulty sensor sends a permanent cold signal to the engine, it can also send a permanent hot signal.

How do I tell if my thermostat is bad?

7 Ways to Tell Your Thermostat Is Broken
  1. The Thermostat Has No Power. ...
  2. The AC or Furnace Won't Turn On. ...
  3. The Heater or AC Won't Turn Off. ...
  4. The Thermostat Reading Doesn't Match the Room Temperature. ...
  5. The Thermostat Doesn't Respond. ...
  6. The HVAC System Is Short Cycling. ...
  7. The Thermostat Forgets Programmed Settings.

Will the P0128 code clear itself?

This is referred to as a permanent code. This permanent code will go away on its own after the monitor runs and passes. Until then it will be present but only when read with a scan tool and will not turn the light on. If the problem Is detected again by the computer it will first set a pending code.

What is the obd2 code for overheat?

P0217 Code: Engine Coolant Over Temperature Condition. Diagnostic trouble code (DTC) P0217 stands for “Engine Coolant Over Temperature Condition.” Code P0217 sets when the engine overheats.

Will a water pump throw a code?

If your car's water pump goes out, one or more warning lights may pop up on the dash. For example, you may see the low coolant warning, overheat warning, or a check engine light.

How long should a thermostat last in a car?

A car thermostat is designed to last a long time – typically around 100,000 miles or 10 years. However, various factors can impact its lifespan. First, driving conditions play a role. If you regularly drive in hot weather or stop-and-go traffic, your thermostat may suffer additional stress.

How can I test my thermostat?

One easy thing you can do to check the accuracy of your thermostat is to do a temperature check. See what your thermostat reads. Then use a portable indoor thermometer to compare the temps. Another good way to do the temperature check is to download one of the many free apps that allow you to check indoor temperatures.

How do I know if I need a new temperature sensor?

Symptoms of a bad coolant temperature sensor:

Engine Overheating or Overcooling: A malfunctioning coolant temperature sensor can result in improper engine cooling. If the sensor provides a falsely low reading, the ECU may not activate the cooling fans or engage the radiator properly, leading to engine overheating.

Is it bad if your car is on C?

“C” on the car's dashboard indicates the automatic air conditioning system used to cool the vehicle. But when the needle stays on “C” for a long time, the system has a problem. The following are the reasons for this problem: Faulty temperature sensor.

What happens if I unplug coolant temperature sensor?

It's not recommended to unplug the coolant temperature sensor unless necessary for diagnostic purposes, as it can adversely affect engine operation and potentially cause damage if left unplugged for an extended period.

Can a car run with a bad thermostat?

Driving with a bad thermostat is like playing Russian roulette with your engine. The longer you drive with a faulty thermostat, the more you risk causing significant damage to your car. The engine might overheat, causing the head gasket to fail or even leading to a cracked engine block.

How to tell if a water pump is bad?

Symptoms of a Failing Water Pump

Overheating is an obvious symptom of a failing water pump. Drivers should make sure the needle on the engine temperature gauge doesn't go into the red. A warning light should also illuminate to tell the driver the coolant is getting too hot.

What happens to engine when thermostat is not working?

Due to corrosion or aging, your car's thermostat can get stuck in a closed position. If this happens, the thermostat will not let the coolant reach the radiator, and, as mentioned above, the engine will overheat, causing severe damage. One of the lesser-known issues is the polar opposite of overheating.

What are the symptoms of a bad thermostat in a car?

Accurate Auto lists these signs below.
  • Odd Temperature Gauge Readings. Your temperature gauge should register cold when you first start your car and slowly build up to normal as the engine warms up. ...
  • Air Temperature Changes. ...
  • Coolant Leaks. ...
  • Strange Sounds. ...
  • Heater Problems.

How do I know if my AC temperature sensor is bad?

If your AC shows irregular temperatures, where sometimes it's much cooler than other times, this can indicate AC sensor malfunction. This, in turn, will lead to AC sensors needing replacement.

What are the symptoms of a bad coolant level sensor?

Coolant level sensors report low radiator coolant. Symptoms of a bad coolant level sensor include an illuminated check engine light, an untriggered coolant level sensor, and an overheating engine.

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