How to tell if your thermostat or water pump is bad?

Author: Misael Friesen  |  Last update: Tuesday, August 19, 2025

The most common symptom of a bad water pump is a leak. Do you have a leak? The most common symptom of a bad thermostat is overheating. A bad water pump can't (except very rare circumstances) cause over heating, and a bad tstat doesn't cause a leak.

What are the symptoms of a bad thermostat?

What Are the Signs of a Bad or Broken Thermostat?
  • The AC or Furnace Won't Come On. ...
  • The HVAC System Won't Turn Off. ...
  • The Thermostat Is Unresponsive. ...
  • The HVAC System Short-Cycles. ...
  • The Room Temperature Doesn't Match the Thermostat Setting. ...
  • The Programmed Settings Reset Themselves. ...
  • Check and Adjust the Settings.

How do you diagnose a bad water pump?

If you notice coolant leaking from your vehicle, especially if it is coming from the front or back of the engine, it's a good idea to have your water pump checked out by a mechanic. Another common symptom of a failing water pump is an overheating engine.

How to check a water pump without removing it?

Since you cannot physically inspect the impeller and inside of a water pump without removing the pump, the best way to know if it is functioning properly is to visually inspect the coolant condition, test the coolant pH and protection levels, test drive the vehicle to verify normal operating temperature, and squeeze ...

Should I change the water pump and thermostat?

Symptoms of Failure: If you are experiencing overheating or cooling issues, it might be indicative of thermostat failure, making replacement necessary. In summary, while you don't have to replace the thermostat when replacing a water pump, doing so can be beneficial for long-term reliability and efficiency.

SYMPTOMS OF A BAD WATER PUMP

How to tell if a thermostat or water pump is bad?

Here are signs you've got a bad pump or thermostat.
  1. Corroded Cooling System Parts. Both of these cooling system parts can get corroded if they start to leak slowly. ...
  2. Leaking Engine Coolant. ...
  3. Erratic Temperature Changes. ...
  4. Poor Coolant Circulation. ...
  5. Overheating Constantly.

Can you replace the thermostat without replacing water pump?

A defective water pump cannot be repaired, it must be replaced with a new one. It is a common practice to also replace the vehicle's thermostat when doing a water pump replacement. It is not always needed, but if you suspect that your engine got hot when the water pump failed, it's a good idea to take care of it.

How to check a thermostat?

Methods for Testing Your Thermostat
  1. Just Turn it On and Let it Work. One way to test your thermostat is to simply turn it on and see if it cools the room down. ...
  2. Get an Indoor Thermometer. Another way to test your thermostat is to use a thermometer. ...
  3. Use a Multimeter. ...
  4. What to Do if You Notice an Issue.

How to tell if a water pump needs replacing?

6 signs your water pump is failing ​
  1. Coolant Leak. The water pump has several gaskets that can be damaged or worn out over time. ...
  2. Engine Overheating. ...
  3. High Pitched, Harmonic Whining Noises. ...
  4. Water Pump Rust and Corrosion. ...
  5. Steam comes out from under your hood. ...
  6. Holes or leakage system on the dry side of the water pump.

Will a bad 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 to check if coolant is circulating?

Take your radiator cap off and note the coolant level. idle the vehicle until it's hot enough for the fans to kick on, maybe 10-15mins depending. When they do, you can look down in your radiator and see the coolant level drop and probably see it circulating as well.

How do I know if my car needs a new thermostat?

5 Signs Your Thermostat Is Broken
  1. You Notice Erratic Temperature Changes. ...
  2. You Find Incorrect Readings on the Temperature Gauge. ...
  3. Your Vehicle Makes Concerning Noises. ...
  4. You Find Coolant Leaks. ...
  5. You Notice Problems with the Heating System.

What are the symptoms of a bad head gasket?

Common Signs of a Bad Head Gasket
  • Coolant Loss: Keep an eye on your coolant levels — a significant drop is a red flag. ...
  • White Smoke from the Tailpipe: Some white smoke from the exhaust pipe of a cold engine is normal. ...
  • Engine Knocking: Blown head gaskets can lead to poor compression. ...
  • Milky Oil: Check your engine's oil.

Is there a code for a bad thermostat?

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

How do I know if my thermostat is faulty?

Signs of a Bad Thermostat

If there's no change in your home's temperature when you adjust the temperature, or the display is unlit or unresponsive, your thermostat may be broken, or the batteries need replacing.

How to test a car water pump?

After, check if the water pump functions by the coolant rushing out after removing the radiator's blockage. If that doesn't happen, use a rag to squeeze into the upper radiator hose while someone steps on the accelerator. Then, release the radiator hose to check for coolant flow.

What are the signs of a water pump?

Signs Your Water Pump is Failing
  • Leaking coolant. A coolant leak can indicate many failures, but if it's coolant on the front of the engine, it's often a water pump leak. ...
  • Overheating. ...
  • Squealing noise. ...
  • Whining noise. ...
  • Water pump corrosion.

What does a bad water pump sound like when idling?

A whining or groaning noise usually means that either a drive belt is loose, or the water pump pulley is bad. It may surprise your customer that a pulley could fail, but it does occasionally happen. Pulleys have been known to fail due to rust, which can cause cracking between their bolt holes.

Can a water pump be bad without leaking?

One of the common signs of bad water pump functionality is overheating. The pump may not leak coolant, but the pump mechanism itself may have failed. Over time the fins of the pump can actually wear down to the point where they are no longer moving coolant.

How much does it cost to replace a water pump and thermostat?

A water pump replacement costs between $400 and $800 depending on the vehicle and the difficulty involved. Parts can run from $200 to $400, with labor services similarly priced. These estimates do not include taxes or shop materials, and disposal fees.

Do you have to drain all coolant when changing thermostat?

Loosen the top hose clip, then ease off the hose. Partially drain the cooling system first. With a normal top-mounted thermostat you need drain only part of the cooling system .

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