Weak spring: Thermostats rely on a spring to open and close correctly. If the spring is of lower quality, it'll weaken prematurely and fail to close the thermostat. As a result, the thermostat remains stuck in the open position.
As the engine reaches its operating temperature, the thermostat gradually opens, allowing coolant to flow to the radiator.
Generally, repairing a thermostat costs around $200 to $500. This is the price range you'll find for most vehicles, although some may be higher or lower depending on the circumstances. But if we're talking complete replacement, expect the price to increase by $50.
If stuck open, the thermostat will allow coolant to continue to flow into the engine, even when it doesn't require more coolant. If this happens, and you turn the heater on simultaneously, then only cool air will come out of the vents, no matter how high you turn up the heat.
If your thermostat is stuck open, you will experience poor performance and fuel economy. If you are running your heater, you may also notice that it doesn't get as hot as it should.
If this doesn't work, try resetting your thermostat with the reset button, changing the batteries, or cleaning off any dust and dirt inside. If the temperature is still stuck, an electrical or display issue might be to blame. Contact an HVAC professional for further help.
Power problems are one of the most common causes of thermostat failure. Whether your thermostat is wired to the mains or runs on batteries, a lack of power will stop your thermostat from accepting commands and communicating with your HVAC system.
Causes of a P0128 code include:
A thermostat that's malfunctioning or stuck open (electronically controlled or traditional)
Replacing a Thermostat. You'll pay between $15 and $300 for the unit in addition to labor ($65 to $100 per hour) to replace an old thermostat with a new one. However, installing a brand-new thermostat is more involved and, therefore, more expensive.
When the thermostat is stuck in the open position, the coolant will continue to circulate. The engine will also not be able to reach normal operating temperature. In this case, the fuel atomization effect is poor, which can also easily lead to misfires.
You can try to clean or knock on a thermostat to get it unstuck. However, that's only a short-term fix you should not rely on. Remember: a stuck thermostat is terrible news for your engine, regardless of whether it's stuck open or closed. So, the best thing you can do is replace your stuck thermostat with a new one.
This could be due to electrical problems, a software glitch, or physical damage. Some thermostats have a lock feature to prevent unauthorized changes. If this feature is enabled, you'll need to unlock the controls before you can adjust the temperature settings.
If the temperature is set to 74°F but the thermometer shows 78°F, you will have to recalibrate the thermostat to account for this 4-degree deviation. Even with older AC units or furnaces, you might have to recalibrate your thermostat from time to time.
When your thermostat doesn;t respond to temperature conditions in your cooling system or gets stuck in either the open or closed position, it may cause a Check Engine Light to turn on. Ensure your thermostat is in good condition in order to prevent engine damage from excess heat.
If you drive with a bad thermostat, your car could overheat, causing severe and expensive engine damage. And while it's technically possible to drive without a thermostat, we don't recommend it. Your car will not reach an acceptable operating temperature, resulting in higher fuel consumption and excessive engine wear.
While there is a small chance that there is an issue with the running temperature of your engine, by far the most common issue with trouble code P0128 is a faulty thermostat. The exact location of the thermostat and issues related to a damaged thermostat may vary depending on your vehicle.
Most modern thermostats will have a clearly marked reset button inside the panel. It's usually small and recessed, so you might need a pointed object like a paperclip to press it. For those wondering specifically how to reset air conditioner thermostat models, the process is generally the same.