A bad car thermostat can cause engine overheating or overcooling, HVAC (heating, ventilation, air conditioning) issues, and poor fuel economy. You might also notice coolant leaks, rumbling noises, and inconsistent temperature gauge readings.
Here are the top bad thermostat symptoms:
- An active check engine light: When the car’s computer notices inconsistent engine temperature conditions, it illuminates the check engine light to warn you.
- HVAC vents blow warm air: A stuck-closed thermostat can cause engine overheating, which may cause the air conditioner to blow warm air or shut off to protect the engine.
- Heater problems: If your car heater fails to produce enough heat, the thermostat may be non-functional.
- Coolant leak: A jammed thermostat causes the coolant to overheat and expand. This creates excess pressure in the cooling system, leading to leaks from hoses or the radiator.
- Temperature fluctuations: If you notice excessive temperature fluctuations on the dashboard gauge, a faulty thermostat might be to blame.
- Strange rumbling or knocking sounds: Unusual knocking or rumbling sounds from the engine bay can be a symptom of a malfunctioning thermostat.
- Overheating or overcooling: The thermostat regulates the engine temperature. If it’s faulty, it can cause the engine to overheat or run too cool.
- Decreased fuel efficiency: A thermostat stuck open may force the engine to consume more gas.
When you notice one or more of these bad thermostat symptoms, you can call AutoNation Mobile Service for a thorough diagnosis and repair right from your driveway.
Want to know more about the symptoms before getting them fixed?
In this guide, we cover all eight signs in depth, explore why you should resolve them fast, and discuss the costs of installing a new thermostat.
This Article Contains:
- 8 Bad Thermostat Symptoms to Look Out For
- How to Diagnose a Faulty Thermostat
- How to Replace a Bad Thermostat: A General Know-How
- How Much Does a Thermostat Replacement Cost?
- 4 FAQs about Thermostats
8 Bad Thermostat Symptoms to Look Out For
Common signs of a malfunctioning thermostat include:
1. Illuminated Check Engine Light
When the thermostat stops working, it fails to regulate the engine temperature, triggering the check engine light on the dashboard.
2. HVAC System Vents Blow Warm Air
A sudden cabin temperature change (from cold to hot) is a clear sign of thermostat failure. This occurs when the faulty thermostat stays shut, preventing coolant from flowing to the radiator and cooling the engine.
In some modern cars, as the engine overheats, the computer automatically shuts off the air conditioning compressor, which causes the AC to blow warm air.
3. Heater Problems
This is a common issue in winter. It occurs when a faulty thermostat valve is stuck open and the coolant keeps circulating, preventing the engine from reaching an optimal temperature.
As a result, the heating system won’t get enough hot coolant to warm the cabin, causing the HVAC vents to blow ambient or cool air instead.
4. Coolant Leak
Coolant can’t circulate properly when your thermostat stays closed, causing it to overheat and build pressure inside the cooling system. This increased pressure can lead to coolant leaks from weak points around the radiator cap and hoses.
5. Bad Temperature Gauge Readings
Your engine coolant temperature gauge (coolant temp gauge) can help you spot a broken thermostat.
If you frequently notice high or low temperature readings, it means the thermostat isn’t regulating the engine’s normal operating temperature. Maintaining optimal temperature is critical for engine performance, as inconsistent temperature levels can cause long-term damage.
6. Rumbling or Knocking Noises
If your thermostat is stuck closed and not functioning correctly, it can prevent coolant from circulating through the radiator. This leads to engine overheating, which can cause the coolant to reach its boiling point.
As a result, you may hear knocking or rumbling noises from the engine bay, especially when you accelerate. These sounds often indicate abnormal combustion due to excessively high temperatures in the combustion chamber.
7. Overheating or Overcooling
If the thermostat valve gets stuck in a closed position due to corrosion or aging, it won’t let hot coolant reach the radiator to dissipate heat. This can cause the engine’s temperature to rise to dangerously high levels.
In contrast, if a thermostat valve gets stuck open, the coolant will flow continuously to the radiator and cause overcooling.
8. Increased Fuel Consumption
As mentioned above, thermostat failure means the engine won’t run at its appropriate operating temperature, leading to either engine overheating or overcooking. In both situations, the fuel efficiency will drop, and exhaust emissions will rise.
If you’re noticing the above signs of a failing thermostat, let’s see how to confirm it.
How to Diagnose a Faulty Thermostat
Diagnosing a malfunctioning thermostat is complicated and risky. You have to deal with hot engine components and perform a series of diagnostic steps to identify the issue.
This can be demanding for inexperienced people and is best left to a professional mechanic.
Here’s how they’ll verify a failing car thermostat:
- Monitor the temperature gauge: They’ll start the engine and observe the temperature gauge. If it steadily rises into the normal range (195°F to 220°F), it indicates proper thermostat function. But if they notice rapid overheating, temperature drops, or slow warm-up, it may suggest a stuck or failing thermostat.
- Scan for error codes: They’ll use an OBD-II scanner to check for codes like P0128 (faulty engine coolant temperature).
- Listen for noises: If they hear rumbling or knocking noises from the engine bay, it could indicate the thermostat is stuck closed and restricting coolant flow.
- Feel the radiator hose: Next, they’ll carefully touch the upper radiator hose when the engine is warm:
- If the hose is cold, the thermostat may be stuck closed.
- If it becomes hot too fast, the thermostat may be stuck open.
- Use an infrared thermometer: If they don’t want to touch the hot hose, they’ll aim an infrared thermometer at the thermostat housing to measure the temperature change. If the temperature jump is around 195°F to 220°F, the thermostat may be working, but a delayed reading may point to a failing unit.
- Check for leaks: Inspect the thermostat housing, nearby hoses, and radiator cap for a coolant leak.
NOTE: Another common method a mechanic may follow is submerging the bad thermostat in a boiling pot of water. If the thermostat opens, there’s no issue, but you’ll have to replace it if it stays closed.
Keep reading to know how it’s done.
How to Replace a Bad Thermostat: A General Know-How
Once your mechanic confirms a bad thermostat is the culprit, here’s how they’ll replace it:
- Let the engine cool down completely and remove the radiator cap.
- Keep a large container or bucket underneath the radiator’s drain plug to collect escaping coolant.
- Turn the drain valve counterclockwise slowly until the fluid starts dripping. Let the fluid drain until the thermostat becomes visible, then turn the drain valve clockwise to stop the flow.
- Remove the clamp that connects the upper radiator hose (coolant hose) to the radiator.
- Disconnect the radiator hose.
- Unscrew the bolts on the thermostat housing and the bad thermostat.
- Take off the gasket around the thermostat housing.
- Remove the broken thermostat.
- Install the new thermostat (spring-side down) and tighten the bolts.
- Reconnect the hose and clamp (use a new coolant hose if the current one is old or damaged).
- Add the coolant back into the radiator reservoir (if it isn’t old or contaminated) and fix the cap back in place.
- Start the engine and let it run for 10 minutes. This will remove the air bubbles in the coolant system.
- Let the engine reach the optimal temperature, then use an infrared thermometer to check the temperature near the thermostat housing. After installing a new thermostat, the temperature should be around 195°F to 220°F.
Wondering how much this repair service will cost?
We list the estimated expenses next.
How Much Does a Thermostat Replacement Cost?
You’ll roughly spend around $480 to $570 to replace a thermostat, which includes:
- Parts cost: $350 to $400
- Labor cost: $130 to $170 per hour
Still have questions about the replacement?
We’ve got you covered!
4 FAQs about Thermostats
Here are the answers to some queries you may have about car thermostats:
1. What Does a Thermostat Do?
A thermostat is part of your vehicle’s cooling system. It regulates coolant flow to help your car’s engine reach and maintain a normal operating temperature.
When the engine is cold, the thermostat stays closed, allowing it to warm up quickly. Once the engine heats up, the thermostat opens to let coolant flow to the radiator and dissipate heat, keeping your engine within the safe temperature range.
However, a failing car thermostat won’t function as needed and will fail to maintain an appropriate operating temperature.
2. Can You Drive with a Bad Thermostat?
You technically can, but you shouldn’t drive with a bad thermostat. Without a working car thermostat, your engine will either overheat or stay cold, hampering vehicle performance.
If you continue to drive with a faulty thermostat for prolonged periods, your engine may fail prematurely, leading to costly repairs.
3. How Often Should You Replace a Car Thermostat?
You should swap out the thermostat every 100,000 miles or 10 years.
However, your thermostat may malfunction earlier because of corrosion, frequent stop-and-go driving, repeated coolant exposure, etc.
4. How Do You Prevent a Thermostat Problem?
Some handy tips that can help extend your thermostat’s lifespan are:
- Change and refill the coolant system every two to three years or around 60,000 miles.
- Use vehicle-compatible coolant for your car.
- Look out for erratic temperature gauge readings to catch a failing thermostat early.
- Schedule a cooling system check without fail once a year before summer.
Get Fast and Reliable Thermostat Repairs with AutoNation Mobile Service
Promptly replacing a malfunctioning thermostat will help your engine last longer and save you from expensive repairs. So, if you notice bad car thermostat symptoms, take your vehicle for inspection immediately.
Don’t want to risk driving an overheated car to the garage?
Let the experts at AutoNation Mobile Service come to you!
We’re a mobile auto repair and maintenance company that offers top-quality service with a 12-month, 12,000-mile repair warranty.
Contact us for a quick and smooth thermostat replacement.